DCL09-A. Declare functions that return an errno with a return type of errno_t

Many existing functions that return an errno are declared as returning a value of type int. It is semantically unclear by looking at the function declaration or prototype if these functions return an error status or a value (or worse, some combination of the two).

TR 24731-1 defines a new type of errno_t that is type int in <errno.h> and elsewhere. Many of the functions defined in TR 24731-1 return values of this type. As a matter of programming style, errno_t should be used as the type of something that deals only with the values that might be found in errno. For example, a function that returns the value of errno should be declared as having the return type errno_t.

Non-Compliant Code Example

This non-compliant code example illustrates a function called opener() that is declared as returning a value of type int. The function, however, uses this return value to indicate the return status of the function by returning values of errno. Consequently, the meaning of the return value is not as clear as it could be.

enum { NO_FILE_POS_VALUES = 3 };

int opener(FILE* file, int *width, int *height, int *data_offset) {
  int file_w;
  int file_h;
  int file_o;
  fpos_t offset;

  if (file == NULL) { return EINVAL; }
  if (fgetpos(file, &offset) != 0) { return errno; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != NO_FILE_POS_VALUES) { return EIO; }
  if (fsetpos(file, &offset) != 0) { return errno; }

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0;
}

Compliant Solution

In this compliant solution, the opener() function returns a value of type errno_t, providing a clear indication that this function returns an errno.

#include <errno.h>

enum { NO_FILE_POS_VALUES = 3 };

errno_t opener(FILE* file, int *width, int *height, int *data_offset) {
  int file_w;
  int file_h;
  int file_o;
  int rc;
  fpos_t offset;

  if (file == NULL) { return EINVAL; }
  if (fgetpos(file, &offset) != 0 ) { return errno; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != NO_FILE_POS_VALUES) { return EIO; }
  if (fsetpos(file, &offset) != 0 ) { return errno; }

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0;
}

NOTE: EINVAL and EIO are not defined in C99, but they are available in most implementations and are defined in Posix.

Risk Assessment

Failing to test for error conditions can lead to vulnerabilities of varying severity. Declaring functions that return an errno with a return type of errno_t will not eliminate this problem, but may reduce errors caused by programmers misunderstanding the purpose of a return value.

Related Vulnerabilities

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

References

[[ISO/IEC TR 24731-1-2007]]
[[ISO/IEC 9899-1999]] Section 6.7.5.3, "Function declarators (including prototypes)"
[[Open Group 04]]