FIO40-C. Reserved

title - reset strings after fgets failure

According to C99, if the fgets() function fails, the contents of its parameterized array are undefined. Therefore, 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.

char buf[1024];
FILE *fptr;
/* fptr initialized to some file */

if(fgets(buf, 1024, fptr) == NULL) {
  /* set error flag and continue */
}
printf("Read in: %s\n", buf);

Compliant Solution

After fgets fails, buf is set to an error message.

char buf[1024];
FILE *fptr;
/* fptr initialized to some file */

if(fgets(buf, 1024, fptr) == NULL) {
  /* set error flag and continue */
  strcpy(buf, "fgets failed");
}
printf("Read in: %s\n", buf);

Risk Assessment

Making assumptions about the contents of the array set by fgets on failure could lead to undefined behavior, possibly resulting in abnormal program termination.

References

[[ISO/IEC 9899-1999:TC2]] Section 7.19.7.2, "The fgets function"