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According to C99, if the If either of the C Standard fgets() function fails or fgetws() functions fail, the contents of its parameterized array are undefined. Therefore, the array being written is indeterminate. (See undefined behavior 175.)  It is necessary to reset the string to a known value to avoid possible errors on subsequent string manipulation functions.

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Noncompliant Code Example

In this noncompliant code example, an error flag is set upon if fgets() failure fails. However, buf is not reset , and will have unknown contents.has indeterminate contents:

#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };
void func(FILE *file) {
  char buf[BUFFER_SIZE];

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

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

Compliant Solution

After fgets failsIn this compliant solution, buf is set to an error messageempty string if fgets() fails. The equivalent solution for fgetws() would set buf to an empty wide string.

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

if#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };

void func(FILE *file) {
  char buf[BUFFER_SIZE];

  if (fgets(buf, 1024sizeof(buf), fptrfile) == NULL) {
    /* setSet error flag and continue */
    strcpy(*buf, "fgets failed")= '\0';
}
printf("Read in: %s\n", buf);
  }
}

Exceptions

FIO40-C-EX1: If the string goes out of scope immediately following the call to fgets() or fgetws() or is not referenced in the case of a failure, it need not be reset.

Risk Assessment

Making invalid assumptions about the contents of the an array set modified by fgets on failure could lead to undefined behavior, possibly resulting in () or fgetws() can result in undefined behavior 175 and abnormal program termination.

Automated Detection

References

Related Vulnerabilities

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

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.7.2, "The {{fgets}} function"