FIO40-C. Reset strings on fgets() failure

According to C99 [[ISO/IEC 9899:1999]], if the fgets() function fails, the contents of the array it was writing to are undefined. As a result it is necessary to reset the string to a known value to avoid possible errors on subsequent string manipulation functions.

Non-Compliant Code Example

In this example, an error flag is set upon fgets() failure. However, buf is not reset, and will have unknown contents.

enum { BUFFERSIZE = 1024 };

char buf[BUFFERSIZE];
FILE *file;
/* Initialize file */

if (fgets(buf, sizeof(buf), file) == NULL) {
  /* set error flag and continue */
}
printf("Read in: %s\n", buf);

Compliant Solution

In this compliant solution, buf is set to an empty string if fgets fails.

enum { BUFFERSIZE = 1024 };

char buf[BUFFERSIZE];
FILE *file;
/* Initialize file */

if (fgets(buf, sizeof(buf), file) == NULL) {
  /* set error flag and continue */
  *buf = '\0';
}
printf("Read in: %s\n", buf);

Risk Assessment

Making invalid assumptions about the contents of an array modified by fgets() can result in undefined behavior and abnormal program termination.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 7.19.7.2, "The fgets function"

      09. Input Output (FIO)       FIO41-C. Do not call getc() or putc() with stream arguments that have side effects