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fsetpos - cppreference.com

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.

​0​ upon success, nonzero value otherwise.

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.

fsetpos with error checking

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    // Prepare an array of FP (floating-point) 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 FP values into array B.
    double B[SIZE];
    fp = fopen("test.bin","rb");
    fpos_t pos;
    if (fgetpos(fp, &pos)) // 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 FP value
    // current position: after reading one f-p value
    printf("%.1f; read count = %d\n", B[0], ret_code); // print one FP value and ret_code
 
    if (fsetpos(fp, &pos)) // 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 FP value
    printf("%.1f; read count = %d\n", B[0], ret_code); // print one FP value and ret_code
    fclose(fp);
 
    return EXIT_SUCCESS;
}

Possible output:

1.0; read count = 1
1.0; read count = 1

• C23 standard (ISO/IEC 9899:2024):

• 7.21.9.3 The fsetpos function (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.21.9.3 The fsetpos function (p: TBD)

• 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

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