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The C Standard, subclause 7.21.8.2 [ISO/IEC 9899:2011], defines the fwrite() function as follows:

Synopsis 

size_t fwrite(const void *restrict ptr, size_t size, size_t nmemb, FILE *restrict stream);

Description

The fwrite() function writes, from the array pointed to by ptr, up to nmemb elements whose size is specified by size, to the stream pointed to by stream. For each object, size calls are 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) is advanced by the number of bytes successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate

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

ssize_t write (int filedes, const void *buffer, size_t size);

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.

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 null-terminated byte string in to a file using the writefwrite() function, always use the size length of the buffer string as the size string plus 1 (to account for the null character) as the nmemb parameter.

Noncompliant Code Example

In the following piece of codethis noncompliant code example, the size of the buffer is stored in size1, but size2 number of characters are written in to the file. If size2 is greater than size1, write() will not stop copying characters at the null character.

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

int main(){
    char *buffer = NULL;
    longsize_t size1,;
size_t size2;
FILE *filedes;

/* Assume size1 int filedes;

    and size2 are appropriately initialized */

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

    error */
}

buffer = (char *)calloc( 1, size1);
    if (!buffer)
 == NULL) {
  /* Handle  return 0;

    write(filedes, buffer, size2error */
}

fwrite(buffer, 1, size2, filedes);

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

    return 0;
}

Compliant

...

Solution

This compliant solution ensures The following code will ensure that the right correct number of characters are written in to the file.:

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

int main(){
     
char *buffer = NULL;
    longsize_t size1,;
size_t size2;
FILE    int *filedes;

/* Assume size1 is appropriately initialized */

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

    error */
}

buffer = (char *)calloc( 1, size1);
    if (!buffer)
 == NULL) {
  /* Handle  return 0;error */
}

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

    size2 = strlen(buffer) + 1;

    write(filedesfwrite(buffer, buffer1, size2, filedes);

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

    return 0;
}

References

Risk Assessment

Failure to follow the recommendation could result in a non-null-terminated string being written to a file, which will create problems when the program tries to read it back as a null-terminated byte string.

Automated Detection

Related Guidelines

Bibliography

...

Image Added Image Added Image Added Wiki Markup\[1\] [http://www.gnu.org/software/libc/manual/html_node/I_002fO-Primitives.html#I_002fO-Primitives]