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 definition does not state that the fwrite() function will stop copying characters into the file if a null character is encountered. Therefore, when writing a null-terminated byte string to a file using the fwrite() function, always use the length of the string plus 1 (to account for the null character) as the nmemb parameter.

Noncompliant Code Example

In this noncompliant code example, the size of the buffer is stored in size1, but size2 number of characters are written 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>
char *buffer = NULL;
size_t size1;
size_t size2;
FILE *filedes;

/* Assume size1 and size2 are appropriately initialized */

filedes = fopen("out.txt", "w+");
if (filedes == NULL) {
  /* Handle error */
}

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

fwrite(buffer, 1, size2, filedes);

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

Compliant Solution

This compliant solution ensures that the correct number of characters are written to the file:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
char *buffer = NULL;
size_t size1;
size_t size2;
FILE *filedes;

/* Assume size1 is appropriately initialized */

filedes = fopen("out.txt", "w+");
if (filedes == NULL){
  /* Handle error */
}

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

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

size2 = strlen(buffer) + 1;

fwrite(buffer, 1, size2, filedes);

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

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

Tool	Version	Checker	Description
LDRA tool suite		44 S	Enhanced Enforcement

Related Guidelines

Bibliography