W3cubDocs

fsetpos

Defined in header `<stdio.h>`
int fsetpos( FILE stream, const fpos_t pos );

Sets the file position indicator and the multibyte parsing state (if any) for the file stream stream according to the value pointed to by pos.

Besides establishing new parse state and position, a call to this function undoes the effects of ungetc and clears the end-of-file state, if it is set.

If a read or write error occurs, the error indicator (ferror) for the stream is set.

Parameters

stream	-	file stream to modify
pos	-	pointer to a `fpos_t` object to use as new value of file position indicator

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.

Example

fsetpos with error checking.

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    /* Prepare an array of f-p values. */
    #define SIZE 5
    double A[SIZE] = {1.,2.,3.,4.,5.};
    /* Write array to a file. */
    FILE * fp = fopen("test.bin", "wb");
    fwrite(A,sizeof(double),SIZE,fp);
    fclose (fp);
 
    /* Read the f-p values into array B. */
    double B[SIZE];
    fp = fopen("test.bin","rb");
    fpos_t pos;
    if (fgetpos(fp,&pos) != 0)      /* current position: start of file */
    {
       perror("fgetpos()");
       fprintf(stderr,"fgetpos() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
       exit(EXIT_FAILURE);
    }
 
    int ret_code = fread(B,sizeof(double),1,fp);   /* read one f-p value */
    /* current position: after reading one f-p value */
    printf("%.1f\n", B[0]);   /* print one f-p value */
 
    if (fsetpos(fp,&pos) != 0)   /* reset current position to start of file */
    {
       if (ferror(fp))
       {
          perror("fsetpos()");
          fprintf(stderr,"fsetpos() failed in file %s at line # %d\n", __FILE__,__LINE__-5);
          exit(EXIT_FAILURE);
       }
    }
 
    ret_code = fread(B,sizeof(double),1,fp);   /* reread first f-p value */
    printf("%.1f\n", B[0]);                    /* print one f-p value    */
    fclose(fp);
 
    return EXIT_SUCCESS; 
}

Output:

1.0
1.0

References

C11 standard (ISO/IEC 9899:2011):

7.21.9.3 The fsetpos function (p: 337)

C99 standard (ISO/IEC 9899:1999):

7.19.9.3 The fsetpos function (p: 303)

C89/C90 standard (ISO/IEC 9899:1990):

4.9.9.3 The fsetpos function