NAME¶

std::fsetpos - std::fsetpos

Synopsis¶

Defined in header <cstdio>
int fsetpos( std::FILE* stream, const std::fpos_t* pos );

Sets the file position indicator and the multibyte parsing state (if any) for the C
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 std::ungetc and clears the end-of-file state, if it is set.

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

Parameters¶

stream - file stream to modify
pos - pointer to a fpos_t object obtained from std::fgetpos called on a stream
associated with the same file

Return value¶

0 upon success, nonzero value otherwise. Also, sets errno on failure.

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¶

// Run this code

#include <cstdio>
#include <cstdlib>

int main()
{
// Prepare an array of floating-point values.
const int SIZE = 5;
double A[SIZE] = {1., 2., 3., 4., 5.};
// Write array to a file.
std::FILE * fp = std::fopen("test.bin", "wb");
std::fwrite(A, sizeof(double), SIZE, fp);
std::fclose(fp);

// Read the values into array B.
double B[SIZE];
fp = std::fopen("test.bin", "rb");
std::fpos_t pos;
if (std::fgetpos(fp, &pos) != 0) // current position: start of file
{
std::perror("fgetpos()");
std::fprintf(stderr, "fgetpos() failed in file %s at line # %d\n",
__FILE__, __LINE__-3);
std::exit(EXIT_FAILURE);
}

int ret_code = std::fread(B, sizeof(double), 1, fp); // read one value
// current position: after reading one value
std::printf("%.1f; read count = %d\n", B[0], ret_code); // print one value and ret_code

if (std::fsetpos(fp, &pos) != 0) // reset current position to start of file
{
if (std::ferror(fp))
{
std::perror("fsetpos()");
std::fprintf(stderr, "fsetpos() failed in file %s at line # %d\n",
__FILE__, __LINE__-5);
std::exit(EXIT_FAILURE);
}
}

ret_code = std::fread(B, sizeof(double), 1, fp); // re-read first value
std::printf("%.1f; read count = %d\n", B[0], ret_code); // print one value and ret_code
std::fclose(fp);

return EXIT_SUCCESS;
}

Output:¶

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

See also¶

fgetpos gets the file position indicator
(function)
ftell returns the current file position indicator
(function)
fseek moves the file position indicator to a specific location in a file
(function)