FIO18-C. Never expect fwrite() to terminate the writing process at a null character

C99 defines the fwrite() function as follows [ISO/IEC 9899:1999]:

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

The 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 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 nitems 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 in to the file. If size2 is greater than size1, write() will not stop copying characters at the null character.

#include <stdio.h>
char *buffer = NULL;
size_t size1, 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.

char *buffer = NULL;
size_t size1, 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. This will create problems when the program tries to read it back as a null-terminated byte string.

Related Guidelines

CERT C++ Secure Coding Standard: FIO18-CPP. Never expect write() to terminate the writing process at a null character

ISO/IEC 9899:1999 Section 7.19.8.2, "The fwrite function"

Bibliography

http://www.opengroup.org/onlinepubs/009695399/functions/fwrite.html

      09. Input Output (FIO)      FIO19-C. Do not use fseek() and ftell() to compute the size of a file