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Many existing functions that return 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. (See recommendation ERR02-C. Avoid in-band error indicators.)
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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; }
errno = 0;
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;
}
errno = 0;
if (fsetpos(file, &offset) != 0) { return errno; }
if (width != NULL) { *width = file_w; }
if (height != NULL) { *height = file_h; }
if (data_offset != NULL) { *data_offset = file_o; }
return 0;
}
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This noncompliant code example , however, does comply with rule nevertheless complies with ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure.
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#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;
fpos_t offset;
if (file == NULL) { return EINVAL; }
errno = 0;
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;
}
errno = 0;
if (fsetpos(file, &offset) != 0 ) { return errno; }
if (width != NULL) { *width = file_w; }
if (height != NULL) { *height = file_h; }
if (data_offset != NULL) { *data_offset = file_o; }
return 0;
}
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NOTE: EINVAL and EIO are not defined in C99the C standard, but they are available in most implementations and are defined in POSIX.
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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.
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CERT C++ Secure Coding Standard: DCL09-CPP. Declare functions that return errno with a return type of errno_t
ISO/IEC 9899:19992011 Section 6.7.56.3, "Function declarators (including prototypes)"
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