fwrite

size_t fwrite( const void *buffer, size_t size, size_t count,
               FILE *stream );

size_t fwrite( const void *restrict buffer, size_t size, size_t count, 
               FILE *restrict stream );

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

Parameters

Return value

The number of objects written successfully, which may be less than count if an error occurs.

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

Example

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
enum { SIZE = 5 };
int main(void)
{
    double a[SIZE] = {1, 2, 3, 4, 5};
    FILE *f1 = fopen("file.bin", "wb");
    assert(f1);
    size_t r1 = fwrite(a, sizeof a[0], SIZE, f1);
    printf("wrote %zu elements out of %d requested\n", r1,  SIZE);
    fclose(f1);
 
    double b[SIZE];
    FILE *f2 = fopen("file.bin", "rb");
    size_t r2 = fread(b, sizeof b[0], SIZE, f2);
    fclose(f2);
    printf("read back: ");
    for(size_t i = 0; i < r2; i++)
        printf("%0.2f ", b[i]);
}

wrote 5 elements out of 5 requested
read back: 1.00 2.00 3.00 4.00 5.00

References

• C11 standard (ISO/IEC 9899:2011):
• 7.21.8.2 The fwrite function (p: 335-336)
• C99 standard (ISO/IEC 9899:1999):
• 7.19.8.2 The fwrite function (p: 301-302)
• C89/C90 standard (ISO/IEC 9899:1990):
• 4.9.8.2 The fwrite function

See also