# fwrite

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< c‎ | io

Defined in header `<stdio.h>` |  |
size_t fwrite( const void *buffer, size_t size, size_t count, FILE *stream ); |  | (until C99)
size_t fwrite( const void *restrict buffer, size_t size, size_t count, FILE *restrict stream ); |  | (since C99)

Writes `count` of objects from the given array `buffer` to the output stream `stream`. The objects are written as if by reinterepreting 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.

### Parameters

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 | - | pointer to the output stream

### 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

Run this code

    #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);
        int r1 = fwrite(a, sizeof a[0], SIZE, f1);
        printf("wrote %d elements out of %d requested\n", r1,  SIZE);
        fclose(f1);

        double b[SIZE];
        FILE *f2 = fopen("file.bin", "rb");
        int r2 = fread(b, sizeof b[0], SIZE, f2);
        fclose(f2);
        printf("read back: ");
        for(int i = 0; i < r2; i++)
            printf("%f ", b[i]);
    }

Output:

    wrote 5 elements out of 5 requested
    read back: 1.000000 2.000000 3.000000 4.000000 5.000000

### 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

printf fprintf sprintf snprintf printf_s fprintf_s sprintf_s snprintf_s (C99)(C11)(C11)(C11)(C11) | prints formatted output to stdout, a file stream or a buffer (function)
fputs | writes a character string to a file stream (function)
fread | reads from a file (function)
C++ documentation for fwrite

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