...
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 C string in to a file using the fwrite() function, always use the size of the buffer string plus 1 (to account for the null character) as the nitems parameter.
Noncompliant Code Example
In the following piece of code, 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.
| Code Block |
|---|
|
#include <stdio.h>
#include <stdlib.h>
int main(){
char *buffer = NULL;
long size1, size2;
FILE *filedes;
/* ...
* Assume size1 and size2 are appropriately initialized
* ...
*/
filedes = fopen("out.txt", "w+");
if (filedes < 0)
return 0;
buffer = (char *)calloc(1, size1);
if (!buffer)
return 0;
fwrite(buffer, sizeof(char), size2, filedes);
free(buffer);
buffer = NULL;
fclose(filedes);
return 0;
}
|
Compliant Code Example
The following code will ensure that the right number of characters are written in to the file.
| Code Block |
|---|
|
#include <stdio.h>
#include <stdlib.h>
int main(){
char *buffer = NULL;
long size1, size2;
FILE *filedes;
/* ...
* Assume size1 and size2 are appropriately initialized
* ...
*/
filedes = fopen("out.txt", "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;
fwrite(buffer, sizeof(char), size2, filedes);
free(buffer);
buffer = NULL;
fclose(filedes);
return 0;
}
|
References
| Wiki Markup |
|---|
\[1\] [http://www.opengroup.org/onlinepubs/009695399/functions/fwrite.html] |
