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In this noncompliant code example, control reaches the end of the checkpass() function when the two strings passed to strcmp() are not equal. This leads to undefined behavior, and various compilers generate code equivalent to the checkpass function returning various values when no return checkpass() is reached.

int checkpass(char *password) {
  if (strcmp(password, "pass") == 0) {
    return 1;
  }
}
/* ... */
if (checkpass(userinput)) {
  printf("Success!\n");
}

...

int checkpass(char *password) {
  if (strcmp(password, "pass") == 0) {
    return 1;
  }
  return 0;
}
/* ... */
if (checkpass(userinput)) {
  printf("Success!\n");
}

Noncompliant Code Example

In this noncomplaint code example, control reaches the end of the the getlen() function when input does not contain the integer delim. As the potentially undefined return value of getlen is later used as an index into an array, this can lead to a buffer overflow.

size_t getlen(int *input, size_t maxlen, int delim) {
  size_t i;
  for (i = 0; i < maxlen; ++i) {
    if (input[i] == delim) {
      return i;
    }
  }
}
/* ... */
size_t i;
int data[] = {1, 1, 1};
i = getlen(data, sizeof(data), 0);
data[i] = userdata;

Noncompliant Code Example

This compliant solution handles the unexpected situation by immediately terminating the program. The correct method for handling this type of error is specific to the application and the type of error (see ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy for more on error handling).

size_t getlen(int *input, size_t maxlen, int delim) {
  size_t i;
  for (i = 0; i < maxlen; ++i) {
    if (input[i] == delim) {
      return i;
    }
  }
  /* The panic function reports the error and terminates the program. */
  panic("Fatal error: Invalid input!\n");
}
/* ... */
size_t i;
int data[] = {1, 1, 1};
i = getlen(data, sizeof(data), 0);
data[i] = userdata;

Implementation Details

When a program containing this noncompliant code example is compiled with -Wall on most versions of the GCC compiler,

size_t getlen(int *input, size_t maxlen, int delim) {
  size_t i;
  for (i = 0; i < maxlen; ++i
#include <stdio.h>

int main(void) {
    if printf("test\n");(input[i] == delim) {
      return i;
    }
  }
}

the following warning is generated

example.c: In function ‘main’‘getlen’:
example.c:512: warning: control reaches end of non-void function

...

References

\[[ISO/IEC 9899:1999|AA. References#ISO/IEC 9899-1999]\] Section 6.9.1, "Function definitions"