std::fseek

Header: <cstdio>

Sets the file position indicator for the file stream stream.

# Declarations

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

# 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.

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). The standard C++ file streams guarantee both flushing and unshifting: std::basic_filebuf::seekoff.

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 <cassert>
#include <cstdio>
#include <cstdint>
#include <fstream>
#include <vector>
 
int main()
{
    std::ofstream("dummy.nfo") << "8 bytes\n"; // create the file
 
    std::FILE* fp = std::fopen("dummy.nfo", "rb");
    assert(fp);
 
    std::fseek(fp, 0, SEEK_END); // seek to end
    const std::size_t filesize = std::ftell(fp);
    std::vector<std::uint8_t> buffer(filesize);
 
    std::fseek(fp, 0, SEEK_SET); // seek to start
    std::fread(buffer.data(), sizeof(std::uint8_t), buffer.size(), fp);
 
    std::fclose(fp);
    std::printf("I've read %zi bytes\n", filesize);
}