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If control reaches the closing curly brace (}) of a non-void function without evaluating a return statement, using the return value of the function call is undefined behavior. (see See undefined behavior 88.).

Noncompliant Code Example

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This error is frequently diagnosed by compilers. (see See MSC00-C. Compile cleanly at high warning levels.).

Compliant Solution

This compliant solution ensures that the checkpass() function always returns a value:

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#include <stddef.h>
 
size_t getlen(const int *input, size_t maxlen, int delim) {
  for (size_t i = 0; i < maxlen; ++i) {
    if (input[i] == delim) {
      return i;
    }
  }
}
 
void func(int userdata) {
  size_t i;
  int data[] = { 1, 1, 1 };
  i = getlen(data, sizeof(data), 0);
  data[i] = userdata;
}

Implementation Details

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(GCC)

Violating this rule can have unexpected consequences, as in the following example:

The following program violates this rule:

#include <stdio.h>

size_t getlen(const int *input, size_t maxlen, int delim) {
  for (size_t i = 0; i < maxlen; ++i) {
    if (input[i] == delim) {
      return i;
    }
  }
}

int main(int argc, char **argv) {
  size_t i;
  int data[] = { 1, 1, 1 };

  i = getlen(data, sizeof(data), 0);
  printf("Returned: %zu\n", i);
  data[i] = 0;

  return 0;
}

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example.c: In function 'getlen'€™:
example.c:12: warning: control reaches end of non-void function

When run None of the inputs to the function equal the delimiter, so when run with GCC 4.45.3 on Linux, control reaches the value of i is 5, as if end of the getlen() function returned 5, which is undefined behavior and in this test returns 3, causing an out-of-bounds write to the data array.

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This compliant solution changes the interface of getlen() to store the result in a user-provided pointer and return an error code to indicate any error conditionsreturns a status indicator to report success or failure. The best method for handling this type of error is specific to the application and the type of error. (see See ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy for more on error handling.).

#include <stddef.h>
 
int getlen(const int *input, size_t maxlen, int delim,
           size_t *result) {
  if (result == NULL) {
    return -1;
  }
  for (size_t i = 0; i < maxlen; ++i) {
    if (input[i] == delim) {
      if (result != NULL) {
        *result = i;
      }
      return 0;
    }
  }
  return -1;
}

void func(int userdata) {
  size_t i;
  int data[] = {1, 1, 1};
  if (getlen(data, sizeof(data), 0, &i) != 0) {
    /* Handle error */
  } else {
    data[i] = userdata;
  }
}

Exceptions

MSC37-C-EX1: According to the C Standard, 5.1.2.23.34, paragraph 1 [ISO/IEC 9899:20112024], "Reaching the } that terminates the main function returns a value of 0." As a result, it is permissible for control to reach the end of the main() function without executing a return statement.

MSC37-C-EX2: It is permissible for a control path to not return a value if that code path is never taken and a function marked _Noreturn is called as part of that code path. For example:

#include <stdio.h>
#include <stdlib.h>
 
_Noreturn void unreachable(const char *msg) {
  printf("Unreachable code reached: %s\n", msg);
  exit(1);
}

enum E {
  One,
  Two,
  Three
};
 
int f(enum E e) {
  switch (e) {
  case One: return 1;
  case Two: return 2;
  case Three: return 3;
  }
  unreachable("Can never get here");
}

Risk Assessment

Using the return value from a non-void function where control reaches the end of the function without evaluating a return statement can lead to buffer overflow vulnerabilities as well as other unexpected program behaviors.

Related Vulnerabilities

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

Automated Detection

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-758 and MSC37-C

Independent( INT34-C, INT36-C, MEM30-C, MSC37-C, FLP32-C, EXP33-C, EXP30-C, ERR34-C, ARR32-C)

CWE-758 = Union( MSC37-C, list) where list =

Undefined behavior that results from anything other than failing to return a value from a function that expects one

Bibliography

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