Reset file pointer c. c o m using System; using System.

Reset file pointer c Im doing some simple fileinput. I want to create a FILE* from a literal string value, so that the resulting FILE* would behave as though there really was a file somewhere that contains the string without actually error: could not reset file handle How to manipulate the file pointer in Perl. See Using fseek with a file pointer that points to stdin – Weather Vane. How do i go about it? Using the same file pointer after resetting it, but ran into a problem. It declares days with an incomplete type. printf( "Enter a string (CR to quit):\n" ); gets(str); strcat(str, "\n" ); /* add a newline */ . after using fgets(a,5,fp1) does the file pointer move 5 positions ahead ? The pointer fp1 is not affected by the fgets call (or any other stdio I/O routine); The FILE object that fp1 points to will be updated to reflect the new However the proper way to close a file, once you've opened it with a stream is to fclose() the FILE pointer. fclose() - close a file. they copy some of the data into memory, in order to optimise access to the actual file. C file pointer assignment to another file pointer. The man page for fdopen specifies the following:. The man page for fclose supports this:. The arguments to fseek() allow you to specify a position relative to the beginning of the file, relative to the end of the file, or A positive value for lDistanceToMove moves the file pointer forward in the file, and a negative value moves the file pointer back. Ask Question Asked 12 years, 7 months ago. You need to use ftell to give you the position within the file. You can't do that dude. Well, I need to read a file (containing only float values) to check how many lines it contains - easy, using fgets. However it starts reading the CSV from the break point rather than the beginning after first run. I am having issues resetting the file read to the start of the file. In the beginning, when we want to open a file to read, we declare a file pointer as follows: FILE *fptr1 = fopen( "filename", "r") I understand that FILE is a data structure in the stdio. You need to dereference your pointer and write the parts of the struct. I also inverted the printf and fgetc statements otherwise EOF will be printed. If you wish, you can simply open the file twice, in which case, the pointers would be entirely independent. I am incrementing the pointer to add new data into the structure. 11. Commented Apr 14, Reset to default 3 . Reading and Writing into a file using pointer in C? 1. You can't treat FILE * like a pointer to memory, it's a pointer to an object of type FILE which in turn holds the state associated with the file I/O. In the above code, what you are returning is the address of the variable that holds fd value. fputs(str, fp); } while (*str!= '\n' ); rewind(fp); /* reset file position indicator to. And actually it works fine, but I always move the pointer starting at the beginning of the file - I use SEEK_SET - it isn't optimized. Sorted by: Reset to default 18 . rewind the position of the file in c; moving to initial position of file in c; c program to get the initial postion of file; DP_File; DP_Pointers; how to move file pointer backwards in C; how to get back to the top of the file in c; file position pointer examples in c; c Resetting an input file pointer with seekg. rewind()) before you do any reads. After I want to display all the records in this structure. Even when I donot do fwrite, just doing fseek is creating problem. On a POSIX system, you get that file descriptor from the open(2) call. How would I be able to reset a pointer to the start of a commandline input or file. I believe that all-bits-zero is a valid representation of 0 for all integer types in all existing C and C++ implementations, which is why the committee was able to add that requirement. When a file is opened with update mode ('+' as How do I reset the file pointer with fgetcsv()? I have the loop below, nested inside another while loop. Takes only the file pointer. *rst++ <<= 3; // etc If the modifications you are making make sense as a functional unit, then you can encapsulate the code in a function call, and avoid declaring rst altogether. FILE is an identifier used as a typedef name, usually for a struct. e. After the while loop ends, the file pointer will be at the end of the file. answered Jul 30, 2020 at 20:35. It's a data structure you use to read and write to/from files. Example of how to use the rewind() function to reset a file pointer back to the beginning of the file in C. Step 2: Declare a file pointer. I do believe the pointer returned by the fopen function is initialized with malloc, so without properly freeing it (using fclose) there will be a memory leak when the program exits There should be no need to call fclose if you never used fopen. In a loop I call a 2 functions that pass in this file pointer to perform 2 different operations on the file. I/O on the stream advances the file position through the file. Answer: Yes, the position of the file pointer is updated This answer doesn't actually address the question that was asked which is simply what happens to the file pointer after calling fread – David Heffernan. Commented May 22, 2012 at 6:53. (There is no similar guarantee for floating-point or pointer types. Share. The std::basic_stringstream allows use of both std::basic_istream::tellg and std::basic_istream::seekg. I am still not fully convinced with this explanation as in some cases (repro cases) the file pointer resetting never happened. BufferedReader read_data_file = new BufferedReader(new FileReader("Datafile. h> FILE * fdopen(int fildes, const char *mode); The fdopen() function associates a stream with the existing file descriptor, fildes. h; Data Structure Algorithm; Data Type; Development; File; Function; Language Basics; Macro Preprocessor; Reset the file reader pointer. That part of memory is allocated at compile time and is read only. positioning the file pointer in a record. You may define days in one file with int days[3];, for example, and declare it in another file with extern int days[];. */ while c) rewind() – Reset File Pointer. Again, we will move the file pointer to the Posted: Tue Apr 15, 2003 4:25 am Post subject: reset file pointer in C++: bare with me, I am kinda learning this as I go What I am planning on doing is reading a textfile with each line an element in an array, as I do not know how many lines this file will have at any given time, I am running through once to count the number of lines, creating I have a C function that reads a stream of characters from a FILE*. I came up with: fstream f; // open long long p = f. For a file pointer in C, how to measure the distance between the current position to the end of I've tried to return a FILE pointer from some function to main(). Re-reading from already read filehandle. The fclose() function shall perform the equivalent of a close() on the file descriptor that is associated with the stream pointed to by stream How to reset to read from beginning of file in C. 4. 18. I can read the beginning of the stick ok, then I try to reset the file pointer to another position and fail. Begin); Here to reset the file pointer you can simply use seek(0) in your code. Try declaring extern char* buffer; in a header and then defining char buffer[256]; in a . error: taking address of temporary Also, note that c++ file pointers not To perform file positioning and seeking in C, include the stdio. In my case I had to show the content in a text window using one pointer and process the data in file using the other pointer. It is not consistent with declaring t_fno as a pointer, leaving it uninitialized, and I have the "while" loop below that will keep reading lines from the file "address. txt. fscanf(), on the other hand, reads what you tell it to, skipping the preceding whitespace. However, after the file location has been reset, whenever I tried to use read(), the return value of read() is always set to -1, which means something was wrong. 0. whence is optional and defaults to 0 which means absolute file positioning, other possible values are 1 If you prefer, you can explicitly reset the shared pointer to make it clear the old object is deleted. To set a position in relation to an EOL (end of line) marker, your code will have Since you're opening the file in text mode, it will convert end-of-line markers, such as LF, into CR/LF. Until this point, my program has been using printf commands, so I was hoping to simply change the commands to fprintf commands, but my compiler is shouting at me because obviously, they are not the same object classes. C++ Open a file and write to arbitrary position of file. It does this by taking an ifstream& object as a parameter, and using getline( ifile, dummystr ) until EOF is hit, at which point the loop breaks and I try to use ifile. Furthermore, the errno message I got was Bad file descriptor. However if I return after the ReadFile call in problemFunction then I exit in the dos->e_magic != If you just want to reset the file pointer to the top of the file, reinitialize your buffer reader. std::ifstream InFile("address. I want to pass the file pointer as an argument to view function. Mark Forums Read If you want to read the file again, you will need to use seekg to change the position of the pointer and then use the clear Ive been trying saving the file pointer location before resetting it and then set it back but things don't seem to work as I want. Remember Me? Forum. Note that the function doesn't close the handle. Load 7 more related questions There's some problem with the global file pointer declaration, as a result of which I'm getting a segmentation fault. However, once the pointer has been incremented, is there a way that I can reset it to point to the first record in the structure? My code is below: According to its docs, f_readdir() expects its second argument to be a "[p]ointer to the file information structure to store the information about read item. This is likely if you're running on Windows (and you probably are, given that your file name starts with "c:\"). start of the file. Since you have reached (and attempted to read past) the end of the file, the eof and (from a file or a string object or any other stream object), you need to call clear before seekg will work as described. I can't think of a reason you'd need a FILE pointer if you weren Sorted by: Reset to default 5 Use fseek() to move the file offset to the desired byte, before using fread(). You can get the current position within a file at any time with ftell(), and reset the current position to arbitrary values with fseek(). Re-creating STDIN Redirection in C. My struct is a big struct with multiple elements. I'm trying to open a file on windows and check that the magic bytes match a windows PE32. nR address before reset: 0x10779f065 nR address after reset: 0x10779f060. \n"); putchar(getc(fp)); rewind(fp); putchar(getc(fp)); fclose(fp); You can get the current position within a file at any time with ftell(), and reset the current position to arbitrary values with fseek(). See File Position. It is indeed a weird thing to do because The rule is very simple: fgets() reads an entire line, unless the length of the line exceeds the provided buffer, in which case it reads as much as it can. txt, moving the file pointer to the end of the written data. GDB gives the following output after executing the first fopen() (gdb) p in_fp $1 = (FILE *) 0x0 (gdb) p *in_fp Cannot access memory at address 0x0 . If you open a file for update (+) and if you do one or more read operations, you must do a positioning operation (e. So for the second call, it has already If your portion is not responsible for deleting pointers then your portion will not want to emulate a std::unique_ptr::reset because that deletes the poiter that the std::unique_ptr is managing. How can one close the stream, but retain the open file descriptor? This behaviour is akin to calling fflush() and then fileno(), and then never using the FILE pointer again, except in closing. The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. Home; C / ANSI-C; assert. C and C++ FAQ; Forum Actions. fclose doesn't set it to NULL because it CAN'T set it to null (it's a pointer to a FILE structure, not to a FILE *). C, Passing a pointer of a file to a function. If your going to declare a file pointer as global in a module, its usually a very good idea to keep it static (contained within that module / object file). How to use an RC circuit and calculate values for a flip flop reset Using PyQGIS to get data contained in the "in-memory editing buffer" of layer that is currently being edited Movie where a woman in an From what I know, rewind will always set the pointer to the beginning of the file. Elastic Stack. How can I delete a file pointed to by a FILE* in C? 2. Writing to/reading from file using pointers, C. Files in C, accessing pointers, reading and writing in files. In FreeBSD, FILE is defined as: If you have already read through the entire file in between the point in your code where you open the file and then wish to "reset the file pointer" you may be running into an issue where you need to clear any flags that may have been set (eof). The file position of a stream describes where in the file the stream is currently reading or writing. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company This functionality of the method is very helpful when you need to move the cursor to a specific or particular location in the file to carry out subsequent read or write operations. Set the line to be read in Perl IO::File. h with "w" mode as it will first close the file and then reopen it for Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company FILE* f = fdopen(d, "w"); man fdopen output: SYNOPSIS. How to use an RC circuit and calculate values for a flip flop reset 0-10V LED Indicator with LM339 Sorted by: Reset to default 7 . FILE *pf; int n = readFile(&pf, "filename. – An fpos_t contains more than the offset: it has information to reset correctly the handling of character sets having a shift state. This code will give you an error, since you can't take the address of a temporary object, as you are doing with &fstream("name. Write to a file in C. Commented Aug 3, 2020 at 19:00. However, as I am using fgets to read in a line of that text file. When fsetpos () is executed, the end-of-file indicator is reset. But now I can only write something after the last line of the original file and don't know how to change the file pointer. j a v a 2 s. You can seek from the start of the file, the end, or from wherever the file offset is currently. If you really care about a general solution, I would recommend building a lookup table for the byte offsets of line endings as you read the file, and then using it to jump around in the file later on. seek(offset[, whence]). Source code: https://github. Your answer is essentialy "yes, but before fclose". Hot Network Questions How does exposure time and ISO affect hue? Pressing electric guitar strings out Sorted by: Reset to default 3 . I have a FILE *, returned by a call to fopen(). The best way to do that is to probably start at the beginnning of the file and simmply read forward until you get to the position, counting the line terminator sequences as you go. You can do it in three (or more) different ways: First is return FILE pointer: Sorted by: Reset to default 7 . 3. I have spent time developing, debugging, and getting visualizations up, and would now like to process all log files in their entirety once again. FILE* file; It's pointer to a file structure. c", after you run the program the file should exist with a size of 5 * sizeof(int) bytes. I am reading in a file and in that file I have to do some operation on each line. seekg( 0, ios::beg ) so I can read the file file is not a pointer to a memory buffer. How might I create a FILE* from a string in this situation?. I believe in main you declared fp as pointer, in your case it wouldn't work because you pass pointer to *fp which means when you open a file pointer to this file is on stack of function open_file, when you get out of function, you loose this pointer. fp is a pointer to a structure FILE. Left aside the fact that you don't check thre return value of fwrite() I would assume that you do write into "test. txt) and try to read it then compiler dynamically allocate memory ( in heap ofcourse because I think internally fopen make use of malloc in order to put file in main memory) and once file stream/data of file is put How do I reset the "file pointer" in filebeats. In C, you can reset to read from the beginning of a file using the fseek() function. Sorted by: Reset to default 0 . Then, rewind() resets the file pointer to the beginning, allowing Resets the file pointer to the beginning of the file. That part of memory still holds "some string\0". The problem is that seekg() sometimes does not work properly How can I keep the position of a FILE pointer in a file in another file pointer? 0. Skip to main content. C Is it possible to change the value being pointed to by a FILE pointer inside a function in C by passing by reference? Here is an example to try and illustrate what I'm trying to do, I can modify the . " This is consistent with the first use you described, wherein you declare t_fno as a local FILINFO structure, and pass its address. Sorted by: Reset to default 37 . The contract you have with fopen is that closing the handle will free all outstanding resources. 2 Undo file/folder operation using java. Modified 2 years, Reset to default 8 . If the input is file (filename received from argv), then I used fseek to get at the end of file, so implementation was somehow easy. Note that your stream must support seeking for this to work; not all streams do. 6. C File pointer returns the contents of a file twice. h; Console; ctype. If Sorted by: Reset to default 3 . Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog Reset the file reader pointer : File Read « File « C / ANSI-C. The file position pointer points to the position in the file, obviously. See POSIX's specification of fopen() for example. FILE wouldn't have worked in K&R C. void rewind(FILE *filePointer); Example: rewind(filePointer); // Move the file pointer to the FILE *fp; printf("Cannot open file. However, the actual inner workings of the object are abstracted away. How to get file pointer value without moving to next file pointer location (in C) 1. Creating own FILE* pointer in C? 6. Here is my code: My function: FILE *create_file(voi Sorted by: Reset to default 2 . If you're really specifically trying to just double the output, and the fd is not a file, omit the fseek() and close the pipe and reopen it and do the read loop the second time. ftell() gives you an offset (but pay attention, its range can be limited to less that the possible file size). FILE* test = fopen("C:\\core. Part of that state is the current position in the file, this doesn't change just because you copy the pointer. Change the function header to. More flexible as it allows moving the This example shows how to read a file, use rewind to reset the position indicator, and read the file again. Looks like doing fseek() to re-set the file-pointer is a bad option in the function above as re-setting the file-pointer to a previous position does not necessarily clear the C-library's own internal buffer. You get a file handle when you open a file by calling fopen, and you use a file handle when you perform explicit read, write, and other operations by calling functions like fread, fprintf, and fseek, and you dispose of a Sorted by: Reset to default 11 . Sorted by: Reset to default 3 Because you are sending a pointer as a parameter, which means it will be created on stack, so the changes you make Your conclusion is correct; once you've reached the end of your stream, you won't be able to read more data until you've reset your position within the stream: myStream. I know, it's a bit subtle. Basically If I run the function twice it won't w. I need to get a file descriptor from it, to make calls like fsync(fd) on it. 0 How to mark a position while reading a file in java?-1 is there any way to reset a previously initialised `bufferedReader` to the beginnig of the stream? Sorted by: Reset to default 108 . If you're interested - pick up a copy of "Expert C Programming: Deep C Secrets" by Peter Van Der Linden - it has a whole chapter about this. So I have a function to detrermine the number of lines in my program. h header file, which provides the necessary functions and definitions. Beats. Sorted by: Reset to default 4 . This is a I opened twice the same file and assigned two pointers and in the end closed both separately. File contents at 2: 2 \n 4 \n 9 \n 5 \n 6 // fp2 is the file pointer for the output file fwrite(fp2, 1, 1, (unsigned)(unsigned char)(*(*buffer))); I get the warnings: Sorted by: Reset to default 1 . and code examples given below to master the concepts and practices of resetting read and write positions in a file. So the functions never have the original values, just copies, and modifying a copy will of course not modify the original. C- Find the current line of my opened text file. Resetting the Read Position to the Beginning So, for eg if i read a line from temp1. For example my function is reading in a line from a file and prints it out using getchar () while((c=getchar())!=EOF) key[i++]=c; if(c == '\n' ) key[i-1] = '\0' printf("%s",key); After running Explanation: Initially, fprintf() writes "Hello, GFG!" to file. extern void adjust_mem(uint8_t *); uint8_t* mem = malloc(10); adjust_mem(mem); In C, I can use rewind back to the start, but I didn't found a similar way in Rust. rewind(fp); Or. The offset is the number of bytes to move the file pointer. Instead, it seems that only the object was deleted, but setting it to zero had no effect (which is what I'd need). And then I want to get the output the data on file from the view function. Files only have a single read pointer. h> int main(int argc, char *argv[]) Tags for rewind - Moves the file position pointer back to the start in C. typedef struct { } FILE; somewhere. offset is the position of the read/write pointer within the file. txt"). `//To read from a file. Ask Question Asked 2 years, 11 months ago. In this case, giving the readBinary function both the responsibility of managing the FILE and reading the data is too much. g. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company I am writing a function which should (if the file already exists) increment the first number by one and append the parameters of the function to the end of the file. Follow answered Sep 7, 2010 I have written content to a file using a file pointer. Both pointers point to the same FILE structure. Basically, as an example, I am reading in a bunch of integers until EOF from an input file. I don't believe there's any way to do this, because a FILE* may not necessarily correspond to a file in the filesystem at all (For example, stdin and stdout). fsetpos () First off, *p++; is wrong. A pointer to the high order 32-bits of the signed 64-bit distance to move. How can I check for EOF in this while loop without moving the file pointer when it checks? Syntax of seek() method fileObject. Sorted by: Reset to default 19 . txt') print file. This means, if you really need some "row" and "column" value: Read the file line by line. Perl using `IO::Handle` or `IO::File` when not reading actual files. BaseStream. txt one by one but it keeps reading the same line from headers. Improve this answer. C Programming - Move to the next line in a text file The linecount function puts the file pointer to the end of the file, after which, there is nothing to read, reset the file pointer to the start of the file after you figure out how many lines there are (or you can close and open the file again). Arguments: Takes three arguments: file pointer, offset, and whence. My initial ideal was to call these two functions; FileStream fs; StreamReader sr;::: fs. The stdio library usually has something like. reassign a file pointer in loop. ReadString(ptr)) { } //here should reset to the beginning of file //re-read the file again I'm writing a program and I want the user to be able to specify whether the output is written to a file or to stdout. fseek(fp, 0L, SEEK_SET); When you call fopen() again you leak resources because you overwrite the pointer and now you can't fclose() the first How to reset file pointer in a CL program? 2 returning to place the FileInputstream had last left. Stack Overflow. Thank you! CFile f; CArchive ar(&f, CArchive::load); while(ar. You shouldn't use free(f) In C, we use a structure pointer of a file type to declare a file: FILE *fp; C provides a number of build-in function to perform basic file operations: fopen() - create a new file or open a existing file. Use the fopen function to open I have a structure and a pointer to point to this structure on the heap. Since FILE * works just fine, there was no need or benefit to redesign the functions to work with FILE instead of FILE * when ANSI C was being drafted. FILE* f is a pointer to a FILE object that is used to identify the stream on all further operations involving file. Hello. txt"); You have the basic idea. If you don file = open('C:\\some_text. how to reset to read from beginning of file in c Posted: Tue Apr 15, 2003 4:25 am Post subject: reset file pointer in C++: bare with me, I am kinda learning this as I go What I am planning on doing is reading a textfile with each line an element in an array, as I do not know how many lines this file will have at any given time, I am running through once to count the number of lines, creating The string you allocated is in the CONST_DATA section (cannot be modified) and hence, you cannot call free on it. As commented, ch should be an int, not a char. There is no reference mechanism in the C language. } } #endif However, the results don't meet my expectations of the object beeing deleted and the pointer beeing reset. The compiler will clearly tell you that types are different. Hot Network Questions Schengen Visa - Purpose vs Length of Stay Explanation: Initially, fprintf() writes "Hello, GFG!" to file. This time when a RCVF operation is conducted it immediately hits the end of file - the message is received straight after it. The implementation of reading functions may use a buffer - i. These system calls don't use the FILE structure to identify a stream, but merely the "file descriptor" from it. Follow edited Jul 30, 2020 at 20:44. Example / / f r o m w w w. h If the file is longer, in most cases (ordinary files on mainstream systems) the remaining bytes will be unchanged. Hello, I used below MFC code to read a file, but I need do that several times, how can I move the pointer to the beginning of file once again. FILE * needs to be a pointer, so in main openReadFile stays as a pointer. Finally, after the –1 value The file pointers denote the location of the file in disk or the processor, and the import section lists the type of DLL files used in the file. If you want to reset the contents of the object without deallocating it and reallocating it, then, provided it has a correctly-implemented assignment operator[1], your idiom of assigning a default-constructed object to it will work *PObject There is no "coordinates" in a file, only a position. If you do one or more write operations, you must do a positioning operation (e. I expected that this will keep on printing the first word of the file in an infinite loop. i. I am new to programming in C, and it looks like printing file pointers is different. Example: append (4,9); append (5,6); File contents at 1: 1 \n 4 \n 9. You can actually do it without using nRreset; since the base address for newResBuffer never changes, you can simply reset it with nR = newResBuffer. – Onofog. You must pass the address of this pointer instead. I want(ed) to implement POSIX tail in C language. h> #include <stdlib. How FP is allocated isn't a concern to you, it's not part of your contract with the API, so don't worry about it. Problem with this code is that it reads the lines from temp1. The offset from the beginning of the file. Change "wb" to "w+b" to allow reading; after writing, you must reset the read-write pointer to the During reading a file, I wanted to reset file pointer to start from the begining of a file. Hello, Windows 10 gcc 5. That should be the job of the owning smart-pointers. Readline on closed filehandle. If the handle is a local static in the The file pointer is then reset to the start of the file with the reset function. Begin); sr. 12. Look up fseek and fsetpos for setting a read/write position in a FILE *. I have declared file pointer fp and assuming that when I use fopen for any file (say test99. You can use the C language union construct to represent an alias for your type as Reset the file pointer to the start in CSharp Description. h in your include path. Using raw pointers is the right thing to do as long as you don't get into deleting/reassigning them. The answer is pretty simple : either call getc() twice; or, as it may be an exercise, you can use fseek(), to move the file pointer to the right location. Is there a way to return the position of the file pointer at a certain moment and use . I used FileStream and StreamReader in C#. Tried to google, didn't get much. 18 File Positioning. Creating fstream object from a FILE* pointer. c file. Read the n1570 draft standard or better The question is if I have to rewind in order to use File pointer from the beginning after using fgetc. So it will look like (don't forget to change the body as well): A file in C is a bunch of bytes, and there is nothing in particular that makes the bytes '\n' and '\r' special (depending on your system). This is not using a file pointer, but may be usefull for you, depending of how your code is working. The following code shows how to reset the file pointer to the start. Miguel13366, you will see my problem if you run the code. 1. After it, I've tried to do some fprintf on the pointer but it wasn't working. But on running I realized it actually prints the whole file. #include <stdio. str() member funciton, a copy of the underlying string is returned in a temporary object, there is no change to the seekpos within the stringstream because you are operating on a copy;; if using For the desired output you can use fseek to reset the fp file pointer. The general rule of thumb is for every alloc have a free. On GNU systems, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. A text file is simply a stream of bytes, and lines are separated by line breaks. All stdio functions dealing with FILE pointers know the contens of and can access the structure members. ) FILE * It's a pointer to a FILE structure, when you call fclose() it will destroy/free FILE structure but will not change the value of FILE* pointer means still it has the address of that FILE structure which is now not exits. The best option for allocating and freeing memory is to do it symmetrically. elastic1622 May 6, 2016, 9:18pm 1. Instead have a single thread be responsible for reading the file, parsing the line, and dispatching jobs. Another thing that should be done is to place a first read ouside the loop otherwise, in the first evaluation, c has yet to have read a character from the file, this can lead to undefined behaviour if c is not yet initialized. It doesn't matter at all whether it's local or global. Using pointer in structure and writing it file in C. ) Sorted by: Reset to default 14 . For example, given a line "1 2 3" (without quotes), fgets() will read the whole line, while fscanf(fp, "%d %d", &i1, &i2) will read Sorted by: Reset to default 16 . Move the file pointer back after fseek. also if you want you can get current pointer location using getFilePointer(). uses both seekp( ) and seekg( ) to reverse the first characters in a file. seekg(); // Save previous location f. And here is where I'm stuck: I need to read the file again to obtain the values and fill the array. The rewind() function sets the file pointer to the beginning of the file. seekg( ) moves the pointer offset number of characters from the specified origin 12. You cannot do that just in standard C++, since iostreams and C I/O are entirely separate and unrelated. By using pointers and indirection, it is somewhat mimicked. same things happem with any pointer getting with malloc. The functions that are exported int readFile(FILE *f, char *fileName) Since you are trying to modify a FILE pointer, you will need to pass a pointer to a pointer to FILE, or FILE **. I believed that file pointer is taken as input only to get the point from where input should be read and fscanf would have a local file pointer which it would use to read the file. What's the function to get a file descriptor from a file pointer? Posted: Tue Apr 15, 2003 4:25 am Post subject: reset file pointer in C++: bare with me, I am kinda learning this as I go What I am planning on doing is reading a textfile with each line an element in an array, as I do not know how many lines this file will have at any given time, I am running through once to count the number of lines, creating We will move the file pointer at the end of the file and write new content; Next, we will move the file pointer at the start of the file and write fresh content at the beginning of the file. It automatically increments to the next line. i need to understand life. I assume that you are also using the try and catch block to check for end of the file. Then another OVRDBF is performed with a different member in this file. The mode of the stream must be compatible with the mode of the file descriptor. How to reset file read pointer in C language. Step 3: Open the file. How can I reset it to the top so I can read in all the lines from the top of the file again? Thank you very much for helping me. Arunbh Yashaswi C originally didn't have the ability to pass structures by value, this was added in ANSI C89. Seek(0, SeekOrigin. fseek()) before you do any writes. I want char *c to begin at 0 position of char *p or just reset the position to position 0 in char *p I can't seem to figure out a way. readlines() When it is run, the first print prints a list containing the text of the file, while the second print prints a blank list. As earlier suggested, it appears the FILE pointer's _flag (as defined under _iobuf) is the key for knowing whether or not a file is read only. The CONST_DATA section, in assembly is like the data-segment (DS), which contains global and static (read-only) variables initialized at A declaration of int days[]; is not a problem by itself. after using fgets(a,5,fp1) does the file pointer move 5 positions ahead ? 7 . txt, i should also read a line from headers. dat"); Otherwise you could use opendir and struct dirent if you want to analyse all the directory and files in a folder. The seek mode. The quick rules of thumb are "Life gets easier if you properly separate responsibilities between functions" and "Life gets complicated if you use static or global variables". The problem is that when the for loop start the file has reached the end, so reads will fail and feof() will return non-zero. int readFile(FILE **f, char *fileName) In the caller, you will need to pass a pointer to a FILE * object:. 3. The fseek() function takes three arguments: 1. Next the values are read back using the read function. But is there a way to 'wind back' the file so that I can read it again? Or is the fastest way just to re-open it? free is called in response to malloc to return allocated memory. Position = 0; This is equivalent to seeking back to the beginning. I think my problem is not as of "Why is “while ( !feof (file) )” always wrong?", because the strings are read properly but I just cannot reset the pointer inputItems to point to the first structure of a structure array. The file pointer p is pointing a structure handled by the C library that manages I/O p is a pointer to the stored FILE object from fopen(). You're going to cause a ridiculous number of race conditions if you try to have multiple threads reading from the same file. You're incrementing the character instead of the pointer. There are a few caveats: if using the . I used fgetc to check, but then it moves the file pointer to the address of the 2nd integer. dat" until end of file. . Takes three arguments: file pointer, offset, and whence. As Kerrek SB points out, another benefit of FILE * is that FILE could be a typedef'd I am trying to understand input and output files in C. I want to open an existed file, and let the file pointer go back to the start point, write new words to it and cover the old one. If I run the code below and return just before the ReadFile call in the function problemFunction the code works fine and it prints 5a4d at the end of the main function. Your mileage may vary though, but the same basic concept should be easily adaptable, example code: Set pointer to current position using lseek(fd, current_position, SEEK_SET); SEEK_SET - set pointer to given offset from the beginning of the file. A Technical Corrigendum added such a guarantee, which is included in the 2011 ISO C standard. Sorted by: Reset to default 8 . By providing (file+1) argument to getc(), you just provided it with an uninitialized area as input. fopen likely indeed does do some mallocing, but the act of closing the handle (fclose) is, by design, going to clean up everything fopen did. Your library cannot access the file system itself, it has to rely on system calls to the kernel. (Or maybe you got lucky with order-of-evaluation and it is indeed incrementing the pointer. 2. Modified 2 years, 11 months ago. Arguments in C is passed by value, and that means they are copied. I do not necessarily have the original filename. when i pass fp, a File pointer, into a new function, and open it there, fclose(fp) causes a seg fault! and i discovered that the file pointer, fp, was never opened. Then I need to malloc a float array to hold the values, also easy. The C programmers must use functions like fopen, feof, ferror, ungetc etc to create and operate on FILE structures. Then, rewind() resets the file pointer to the beginning, allowing fgetc() to read the file from the start, moving the pointer forward one character at a time during reading. Edit: I think my original post may have been misleading. 0 I'm opening a USB memory stick for read. Addenda: fopen is returning a pointer to a FILE Sorted by: Reset to default 3 . So best option is to have structure that will both hold pointer to FILE and to char* with name. The scope of the file pointer has nothing to do with its use. u", "wb"); I am trying to understand what does file pointer increment means . Sorted by: Reset to default 2 . When the stream is closed via fclose(3), fildes is closed also. myfunction C, passing a pointer for a file into a a function as an argument. This article covers the syntax, uses and common examples of rewind() function in C. dat"));' Sorted by: Reset to default 3 . Below is the illustration of the C library rewind () function. If you want the current line, you'll have to count the number of line terminator sequences between the start of the file and the position. I want the file pointer to move to the next line once header_line is read. I have filebeats forwarding logs to logstash/ELK. I read online that you can't open a file to read and write simultaneously. Two options. The arguments to fseek() allow you to specify The fsetpos () function moves the file position indicator of the given file stream to the specified position. C - moving back the pointer in the file using lseek. In my pseudo code current_position is the offset. int a malloc(10); free(a); still a will not be NULL fp isn't allocated by the C runtime, you don't have to free it. That means your files didn't open If a file with the same name already exists its contents are erased and the file is treated as an empty new file. The file pointer. If you do not need the high order 32-bits, this pointer must be set to NULL. closed account . Reply reply aioeu • • Sorted by: Reset to default 7 . How can I move a file pointer to the next line in a file? Files are a collection of bytes, where the meaning of the bytes depend on the file format. Sometimes this is a little easier No, they are not the same. I have a method that takes either a StringReader instance (reading from the clipboard) or a StreamReader instance (reading from a file) and, at present, casts either one as a TextReader instance. Moves the file pointer to a specific location. Well, it wouldn't hurt to include an explanation of that in you It's normal, you need to rewind() the file. IO; static class MainClass { static void Main() { // Create a new file and writer. c o m using System; using System. Technically, truncation is implementation-defined for text files, and no for binary files. You can find this by looking at stdio. I mentioned earlier that there are some easier ways for you to reset the pointer. Flexibility: More flexible as it allows moving the file pointer relative to the beginning, current position, or end. It is really a pointer to the stream to the file, through which operations can be performed. If you open the file in "wb" mode, I suspect you'll find the numbers are identical:. I just quickly set up a simple header file with a template method that accepts any type of pointer. I need it to 'pre-read' some of the source input, then reset the cursor back to the start. The file pointer represents a handle that is used by reading functions like fread() to retrieve data from an actual file (assuming it has been successfully opened, of course). Declare a file pointer variable to store the reference to the file you want to perform positioning and seeking operations on. kk. 1 In this particular case, you could use rst to do your pointer adjustments, and leave mem alone. If the file is already open you can use freopen() function from stdio. Resets the file pointer to the beginning of the file. e. Copying a FILE * makes little sense, and certainly doesn't create a copy of the state in question. com/portfoliocourses You could consider doing a position save and restore: The problem that seems to happen is, my fp->_ptr which actually stores the pointer to the location in the file gets corrupted immediately after doing fseek. if all you want is an object-y way of wrapping a C FILE stream, you could use a unique pointer for that Sorted by: Reset to default 3 . [in, out, optional] lpDistanceToMoveHigh. Your Fopen function changes the value of its local filePtr and You are passing pointer to FILE in the 3rd argument, and you change THE POINTER in the function. readlines() print file. It's really a file descriptor, or a "file handle". Call the clear member function on the stream prior to calling the seekg member function. Is there anyway to reset the file pointer so the subsequent RCVF operations can retrieve the content in the second member? Just a bit of possibly useful information If you open the file as a stream ( fopen()), you have even more flexibility. Not completely unexpected I guess. However, since the question shows days being assigned memory with malloc, either it should be a pointer and not an array or it should be an array and I am currently working with files in c++ and I want to read a file after a certain position. But you can't read from it for two reasons: you open the file write-only. So, when reading a text file you can calculate your "coordinates" if you scan the whole file. seekg(std::ios::beg); // Set ptr to A "file pointer" (or FILE *) is not a "pointer to a file". However, you may actually be wondering if the file will be truncated at that point: usually (same cases) no. I would now like to print this data in the form of an array. cvdji fru kvcu yiomwn evpx wgg yixy gopr gqevpfvkl ifbjgl