Defined in header <cstdio>
std::size_t fwrite( const void* buffer, std::size_t size, std::size_t count, std::FILE* stream );

Writes up to count binary objects from the given array buffer to the output stream stream. The objects are written as if by reinterpreting each object as an array of unsigned char and calling std::fputc size times for each object to write those unsigned chars into stream, in order. The file position indicator for the stream is advanced by the number of characters written.

If the objects are not TriviallyCopyable, the behavior is undefined.

If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.


buffer - pointer to the first object in the array to be written
size - size of each object
count - the number of the objects to be written
stream - output file stream to write to

Return value

Number of objects written successfully, which may be less than count if an error occurred.

If size or count is zero, fwrite returns zero and performs no other action.


#include <cstdio>
#include <vector>
#include <array>
int main ()
    // write buffer to file
    if(std::FILE* f1 = std::fopen("file.bin", "wb")) {
        std::array<int, 3> v = {42, -1, 7}; // underlying storage of std::array is an array
        std::fwrite(v.data(), sizeof v[0], v.size(), f1);
    // read the same data and print it to the standard output
    if(std::FILE *f2 = std::fopen("file.bin", "rb")) {
        std::vector<int> rbuf(10); // underlying storage of std::vector is also an array
        std::size_t sz = std::fread(rbuf.data(), sizeof rbuf[0], rbuf.size(), f2);
        for(std::size_t n = 0; n < sz; ++n) {
            std::printf("%d\n", rbuf[n]);



See also

prints formatted output to stdout, a file stream or a buffer
writes a character string to a file stream
reads from a file
C documentation for fwrite

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