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The C standard defines the fwrite() function as follows

size_t fwrite(const void *restrict ptr

The POSIX standard defines the {{write()}} interface as follows \[1\].

ssize_t write (int filedes, const void *buffer, size_t size, size_t nitems, FILE *restrict stream);

The write function writes up to size bytes from buffer to the file with descriptor filedes. The data in buffer is not necessarily a character string and a null character is output like any other character fwrite() function shall write, from the array pointed to by ptr, up to nitems elements whose size is specified by size, to the stream pointed to by stream. For each object, size calls shall be made to the fputc() function, taking the values (in order) from an array of unsigned char exactly overlaying the object. The file-position indicator for the stream (if defined) shall be advanced by the number of bytes successfully written. If an error occurs, the resulting value of the file-position indicator for the stream is unspecified.

The definition does not state that the writefwrite() function will stop copying characters into the file if a null character is encountered. Therefore, when writing a C string in to a file using the writefwrite() function, always use the size of the buffer string plus 1 (to account for the null character) as the size nitems parameter.

Noncompliant Code Example

...

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(){
    char *buffer = NULL;
    long size1, size2;
    intFILE *filedes;

    /* ...
     * Assume size1 and size2 are appropriately initialized
     * ...
     */

    filedes = openfopen("out.txt", O_CREAT | O_WRONLY"w+");
    if (filedes < 0)
        return 0;

    buffer = (char *)calloc(1, size1);
    if (!buffer)
        return 0;

    writefwrite(filedesbuffer, buffersizeof(char), size2, filedes);

    free(buffer);
    buffer = NULL;
    closefclose(filedes);

    return 0;
}

Compliant Code Example

...

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(){
    char *buffer = NULL;
    long size1, size2;
    intFILE *filedes;

    /* ...
     * Assume size1 and size2 are appropriately initialized
     * ...
     */

    filedes = openfopen("out.txt", O_CREAT | O_WRONLY"w+");
    if (filedes < 0)
        return 0;

    buffer = (char *)calloc(1, size1);
    if (!buffer)
        return 0;

    /* ...
     * Accept characters in to the buffer
     * Check for buffer overflow
     * ...
     */

    size2 = strlen(buffer) + 1;

    writefwrite(filedesbuffer, buffersizeof(char), size2, filedes);

    free(buffer);
    buffer = NULL;
    closefclose(filedes);

    return 0;
}

References

\[1\] [http://www.gnuopengroup.org/softwareonlinepubs/libc009695399/manualfunctions/html_node/I_002fO-Primitives.html#I_002fO-Primitivesfwrite.html]