fseek

Header: <stdio.h>

Sets the file position indicator for the file stream stream to the value pointed to by offset.

# Declarations

int fseek( FILE* stream, long offset, int origin );

#define SEEK_SET /* unspecified */
#define SEEK_CUR /* unspecified */
#define SEEK_END /* unspecified */

# Parameters

stream: file stream to modify
offset: number of characters to shift the position relative to origin
origin: position to which offset is added. It can have one of the following values: SEEK_SET, SEEK_CUR, SEEK_END

# Return value

0 upon success, nonzero value otherwise.

# Notes

After seeking to a non-end position in a wide stream, the next call to any output function may render the remainder of the file undefined, e.g. by outputting a multibyte sequence of a different length.

For text streams, the only valid values of offset are 0 (applicable to any origin) and a value returned by an earlier call to ftell (only applicable to SEEK_SET).

POSIX allows seeking beyond the existing end of file. If an output is performed after this seek, any read from the gap will return zero bytes. Where supported by the filesystem, this creates a sparse file.

POSIX also requires that fseek first performs fflush if there are any unwritten data (but whether the shift state is restored is implementation-defined).

POSIX specifies, that fseek should return -1 on error, and set errno to indicate the error.

On Windows, _fseeki64 can be used to work with files larger than 2 GiB.

# Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    // Prepare an array of double values.
    #define SIZE 5
    double A[SIZE] = {1.0, 2.0, 3.0, 4.0, 5.0};
 
    // Write array to a file.
    FILE * fp = fopen("test.bin", "wb");
    fwrite(A, sizeof(double), SIZE, fp);
    fclose (fp);
 
    // Read the double values into array B.
    double B[SIZE];
    fp = fopen("test.bin", "rb");
 
    // Set the file position indicator in front of third double value.
    if (fseek(fp, sizeof(double) * 2L, SEEK_SET) != 0)
    {
        fprintf(stderr, "fseek() failed in file %s at line # %d\n",
                __FILE__, __LINE__ - 2);
        fclose(fp);
        return EXIT_FAILURE;
    }
 
    int ret_code = fread(B, sizeof(double), 1, fp); // reads one double value
    printf("ret_code == %d\n", ret_code);           // prints the number of values read
    printf("B[0] == %.1f\n", B[0]);                 // prints one value
 
    fclose(fp);
    return EXIT_SUCCESS;
}