The majority of the standard library functions, including I/O functions and memory allocation functions, return either a valid value or a value of the correct return type that indicates an error (for example, −1 or a null pointer). Assuming that all calls to such functions will succeed and failing to check the return value for an indication of an error is a dangerous practice that may lead to unexpected or undefined behavior when an error occurs. It is essential that programs detect and appropriately handle all errors in accordance with an error-handling policy, as discussed in ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy.
The successful completion or failure of each of the standard library functions listed in the following table shall be determined either by comparing the function’s return value with the value listed in the column labeled “Error Return” or by calling one of the library functions mentioned in the footnotes to the same column.
Function | Successful Return | Error Return |
|---|---|---|
| Pointer to space |
|
|
| Nonzero |
|
| Nonzero |
|
| Nonzero |
| Pointer to matching element |
|
| Pointer to matching element |
|
| Converted wide character |
|
| Number of bytes |
|
| Number of bytes |
|
| Pointer to space |
|
| Processor time |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| Nonzero |
|
|
|
|
|
|
| Character read |
|
|
| Nonzero, |
| Pointer to string |
|
| Wide character read |
|
| Pointer to stream |
|
|
| Nonzero |
| Number of characters (nonnegative) | Negative |
| Number of characters (nonnegative) | Negative |
| Character written |
|
| Nonnegative |
|
fputwc() | Wide character written | WEOF |
| Nonnegative |
|
| Elements read | Elements read |
| Pointer to stream |
|
|
| Nonzero |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
|
| Nonzero |
|
| Nonzero, |
| File position |
|
| Number of wide characters (nonnegative) | Negative |
| Number of wide characters (nonnegative) | Negative |
| Elements written | Elements written |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Character read |
|
| Character read |
|
| Pointer to string |
|
| Pointer to string |
|
| Pointer to string |
|
| Wide character read |
|
| Wide character read |
|
| Pointer to broken-down time |
|
| Pointer to broken-down time |
|
| Pointer to broken-down time |
|
| Pointer to broken-down time |
|
| Pointer to space |
|
| Number of bytes |
|
| Number of bytes or status |
|
| Number of bytes or status |
|
| Number of bytes or status |
|
| Number of bytes or status |
|
| Number of non-null elements |
|
|
| Nonzero |
| Number of non-null elements |
|
|
| Nonzero |
| Number of bytes |
|
| Pointer to located character |
|
| Calendar time |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| Number of characters (nonnegative) | Negative |
| Character written |
|
| Wide character written |
|
|
| Nonzero |
| Pointer to space |
|
|
| Nonzero |
|
| Nonzero |
| Pointer to string |
|
|
| Nonzero |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Pointer to previous function |
|
| Number of characters that would be written (nonnegative) | Negative |
| Number of characters that would be written (nonnegative) | Negative |
| Number of non-null characters written | Negative |
| Number of non-null characters written | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Pointer to located character |
|
|
| Nonzero |
| Number of non-null characters |
|
| Pointer to located character |
|
| Pointer to located character |
|
| Pointer to located string |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Pointer to first character of a token |
|
| Pointer to first character of a token |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Length of transformed string |
|
| Number of non-null wide characters | Negative |
| Number of non-null wide characters | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
|
|
|
|
|
|
|
|
|
|
|
|
| Calendar time |
|
| Base |
|
| Pointer to stream |
|
|
| Nonzero |
| Non-null pointer |
|
|
| Nonzero |
|
|
|
| Value of thread-specific storage |
|
|
|
|
| Character pushed back |
|
| Character pushed back |
|
| Number of characters (nonnegative) | Negative |
| Number of characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of wide characters (nonnegative) | Negative |
| Number of wide characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of characters that would be written (nonnegative) | Negative |
| Number of characters that would be written (nonnegative) | Negative |
| Number of non-null characters (nonnegative) | Negative |
| Number of non-null characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of non-null wide characters | Negative |
| Number of non-null wide characters | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of wide characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
| Number of bytes stored |
|
| Pointer to located wide character |
|
| Number of non-null wide characters |
|
| Pointer to located wide character |
|
| Pointer to located wide character |
|
| Number of non-null bytes |
|
|
| Nonzero |
| Pointer to located wide string |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Pointer to first wide character of a token |
|
| Pointer to first wide character of a token |
|
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Number of non-null bytes |
|
|
| Nonzero |
| Converted value |
|
| Converted value |
|
| Converted value |
|
| Length of transformed wide string |
|
| Converted character |
|
| Number of bytes stored |
|
| Number of bytes stored |
|
| Valid argument to |
|
| Valid argument to |
|
| Pointer to located wide character |
|
| Number of wide characters (nonnegative) | Negative |
| Number of conversions (nonnegative) |
|
| Number of conversions (nonnegative) |
|
When FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char) applies, callers shall determine the success or failure of the functions in this table as follows:
1 By calling ferror() and feof()2 By calling ferror()
The ungetc() function does not set the error indicator even when it fails, so it is not possible to check for errors reliably unless it is known that the argument is not equal to EOF. The C Standard [ISO/IEC 9899:2011] states that "one character of pushback is guaranteed," so this should not be an issue if, at most, one character is ever pushed back before reading again. (See FIO13-C. Never push back anything other than one read character.)
Noncompliant Code Example (setlocale())
In this noncompliant code example, the function utf8_to_wcs() attempts to convert a sequence of UTF-8 characters to wide characters. It first invokes setlocale() to set the global locale to "en_US.UTF-8"setlocale() function will fail by returning a null pointer, for example, when the locale is not installed. The function may fail for other reasons as well, such as the lack of resources. Depending on the sequence of characters pointed to by utf8, the subsequent call to mbstowcs() may fail or result in the function storing an unexpected sequence of wide characters in the supplied buffer wcs.
#include <locale.h>
#include <stdlib.h>
int utf8_to_wcs(wchar_t *wcs, size_t n, const char *utf8,
size_t *size) {
if (0 == size) {
return -1;
}
setlocale(LC_CTYPE, "en_US.UTF-8");
*size = mbstowcs(wcs, utf8, n);
return 0;
}
Compliant Solution (setlocale())
This compliant solution checks the value returned by setlocale() and avoids calling mbstowcs() if the function fails. The function also takes care to restore the locale to its initial setting before returning control to the caller.
#include <locale.h>
#include <stdlib.h>
int utf8_to_wcs(wchar_t *wcs, size_t n, const char *utf8,
size_t *size) {
if (0 == size) {
return -1;
}
const char *save = setlocale(LC_CTYPE, "en_US.UTF-8");
if (NULL == save) {
return -1;
}
*size = mbstowcs(wcs, utf8, n);
if (NULL == setlocale(LC_CTYPE, save)) {
return -1;
}
return 0;
}
Noncompliant Code Example (signal())
In this noncompliant example, the function signal() is invoked to install a handler for the SIGINT signal. signal() returns a pointer to the previously installed handler on success and the value SIG_ERR on failure. However, because the caller fails to check for errors, when signal() fails, the function may proceed with the lengthy computation without the ability to interrupt it.
#include <signal.h>
volatile sig_atomic_t interrupted;
void handle_interrupt(int signo) {
interrupted = 1;
}
int f() {
int result = 0;
signal(SIGINT, handle_interrupt);
while (0 == result && 0 == interrupted) {
/* Perform a lengthy computation */
}
/* Indicate success or failure */
return interrupted ? -1 : result;
}
Compliant Solution (signal())
This compliant solution checks the value returned by the signal() function and avoids performing the lengthy computation when signal() fails. The calling function also takes care to restore the disposition for the SIGINT signal to its initial setting before returning control to the caller.
#include <signal.h>
volatile sig_atomic_t interrupted;
void handle_interrupt(int signo) {
interrupted = 1;
}
int f() {
int result = 0;
void (*saved_handler)(int);
saved_handler = signal(SIGINT, handle_interrupt);
if (SIG_ERR == (int)saved_handler) {
/* Indicate failure */
return -1;
}
while (0 == result && 0 == interrupted) {
/* Perform a lengthy computation */
}
if (SIG_ERR == signal(SIGINT, saved_handler)) {
return -1;
}
/* Indicate success or failure */
return interrupted ? -1 : result;
}
Noncompliant Code Example (malloc())
In this noncompliant code example, temp_num, tmp2, and num_of_records are under the control of a malicious user. The attacker can easily cause malloc() to fail by providing a large value for num_of_records.
#include <stdlib.h>
#include <string.h>
typedef struct {
char sig_desc[32];
} signal_info;
void func(size_t num_of_records, size_t temp_num,
const char *tmp2) {
signal_info *start = (signal_info *)malloc(num_of_records *
sizeof(signal_info));
signal_info *point = start + temp_num - 1;
if (tmp2 == NULL) {
/* Handle error */
}
memcpy(point->sig_desc, tmp2, strlen(tmp2));
/* ... */
}
When malloc() fails, it returns a null pointer that is assigned to start. If start is a null, an attacker can provide a value for temp_num that, when scaled by the the sizeof(signal_info), references a writable address to which control is eventually transferred. The contents of the string referenced by tmp2 can then be used to overwrite the address, resulting in an arbitrary code execution vulnerability.
Compliant Solution (malloc())
To correct this error, ensure the pointer returned by malloc() is not null.
#include <stdlib.h>
#include <string.h>
typedef struct {
char sig_desc[32];
} signal_info;
void func(size_t num_of_records, size_t temp_num,
const char *tmp2) {
signal_info *point;
signal_info *start = (signal_info *)malloc(num_of_records *
sizeof(signal_info));
if (start == NULL) {
/* Handle allocation error */
} else if (tmp2 == NULL) {
/* Handle error */
}
point = start + temp_num - 1;
memcpy(point->sig_desc, tmp2, strlen(tmp2));
/* ... */
}
Noncompliant Code Example (realloc())
This noncompliant code example calls realloc() to resize the memory referred to by p. However, if realloc() fails, it returns a null pointer. Consequently, the connection between the original block of memory and p is severed, resulting in a memory leak.
#include <stdlib.h>
void *p;
void func(size_t new_size) {
p = realloc(p, new_size);
if (p == NULL) {
/* Handle error */
}
}
Compliant Solution (realloc())
In this compliant solution, the result of realloc() is assigned to the temporary pointer q and validated before it is assigned to the original pointer p. This solution is also compliant with MEM04-C. Do not perform zero-length allocations.
#include <stdlib.h>
void *p;
void func(size_t new_size) {
void *q;
if (new_size == 0) {
/* Handle error */
}
q = realloc(p, new_size);
if (q == NULL) {
/* Handle error */
} else {
p = q;
}
}
Noncompliant Code Example (fseek())
In this noncompliant code example, the fseek() function is used to set the file position to a location offset in the file referred to by file prior to reading a sequence of bytes from the file. However, if an I/O error occurs during the seek operation, the subsequent read will fill the buffer with the wrong contents.
#include <stdio.h>
size_t read_at(FILE *file, long offset, void *buf,
size_t nbytes) {
fseek(file, offset, SEEK_SET);
return fread(buf, 1, nbytes, file);
}
Compliant Solution (fseek())
According to the C Standard, the fseek() function returns a nonzero value to indicate that an error occurred. Testing for this condition before proceeding to read from the file eliminates the chance of operating on the wrong portion of the file if fseek() fails. Always test the returned value to make sure an error did not occur before operating on the file. If an error does occur, handle it appropriately.
#include <stdio.h>
size_t read_at(FILE *file, long offset, void *buf,
size_t nbytes) {
if (fseek(file, offset, SEEK_SET) != 0) {
/* Indicate error to caller */
return 0;
}
return fread(buf, 1, nbytes, file);
}
Noncompliant Code Example (snprintf())
In this noncompliant code example, snprinf() is assumed to succeed. However, if the call fails (for example, because of insufficient memory, as described in GNU libc bug 441945), the subsequent call to log_message() is likely to result in undefined behavior because the character buffer is not initialized and need not be null-terminated.
#include <stdio.h>
extern void log_message(const char *);
void f(int i, int width, int prec) {
char buf[40];
snprintf(buf, sizeof(buf), "i = %*.*i", width, prec, i);
log_message(buf);
/* ... */
}
Compliant Solution (snprintf())
A compliant solution avoids assuming that snprintf() succeeds regardless of its arguments and tests the return value of the function before using the buffer the function was passed to format output into. In addition, a compliant solution takes care to null-terminate the formatted string in case the provided buffer wasn't large enough for snprintf() to append the terminating null.
#include <stdio.h>
#include <string.h>
extern void log_message(const char *);
void f(int i, int width, int prec) {
char buf[40];
int n;
n = snprintf(buf, sizeof(buf), "i = %*.*i", width, prec, i);
if (n < 0) {
/* Handle snprintf() error */
strcpy(buf, "unknown error");
}
/* Null-terminate buffer in case of overflow */
buf[sizeof(buf) - 1] = '\0';
log_message(buf);
}
When the length of the formatted string is unbounded, it is best to invoke the function twice, once with a NULL buffer to determine the size of output, then dynamically allocate a buffer of sufficient size, and finally call snprintf() again to format the output into the dynamically allocated buffer. Even with this approach, the success of all calls still needs to be tested, and any errors still must be appropriately handled. A possible optimization is to first attempt to format the string into a reasonably small buffer allocated on the stack and, only when the buffer turns out to be too small, allocate one of a sufficient size dynamically. This approach is shown in the following compliant solution.
Note that this solution uses the goto statement, as suggested in MEM12-C. Consider using a goto chain when leaving a function on error when using and releasing resources.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
extern void log_message(const char *);
void f(int i, int width, int prec) {
char buffer[20];
char *buf = buffer;
int n = sizeof(buffer);
const char fmt[] = "i = %*.*i";
n = snprintf(buf, n, fmt, width, prec, i);
if (n < 0) {
/* Handle snprintf() error */
strcpy(buffer, "unknown error");
goto write_log;
}
if (n < sizeof(buffer)) {
goto write_log;
}
buf = (char *)malloc(n + 1);
if (NULL == buf) {
/* Handle malloc() error */
strcpy(buffer, "unknown error");
goto write_log;
}
n = snprintf(buf, n, fmt, width, prec, i);
if (n < 0) {
/* Handle snprintf() error */
strcpy(buffer, "unknown error");
}
write_log:
log_message(buf);
if (buf != buffer) {
free(buf);
}
}
Exceptions
ERR33-EX1: It is acceptable to ignore the return value of a function that cannot fail, or a function whose return value is inconsequential, or if an error condition need not be diagnosed. The function's results should be explicitly cast to void to signify programmer intent. Return values from the following functions do not need to be checked because their historical use has overwhelmingly omitted error checking, and the consequences are not relevant to security.
Function | Successful Return | Error Return |
|---|---|---|
| Character written |
|
| Wide character written |
|
| Nonnegative |
|
| Number of characters (nonnegative) | Negative |
| Number of wide characters (nonnegative) | Negative |
kill_dependency() | The input parameter | NA |
memcpy(), wmemcpy() | The destination input parameter | NA |
memmove(), wmemmove() | The destination input parameter | NA |
strcpy(), wcscpy() | The destination input parameter | NA |
strncpy(), wcsncpy() | The destination input parameter | NA |
strcat(), wcscat() | The destination input parameter | NA |
strncat(), wcsncat() | The destination input parameter | NA |
memset(), wmemset() | The destination input parameter | NA |
Risk Assessment
Failing to detect error conditions can lead to unpredictable results, including abnormal program termination and denial-of-service attacks or, in some situations, could even allow an attacker to run arbitrary code.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
ERR33-C | High | Likely | Medium | P18 | L1 |
Automated Detection
Tool | Version | Checker | Description |
|---|---|---|---|
Can detect violations of this recommendation when checking for violations of EXP12-C. Do not ignore values returned by functions and EXP34-C. Do not dereference null pointers | |||
| 2017.07 | CHECKED_RETURN | Finds inconsistencies in how function call return values are handled. Coverity Prevent cannot discover all violations of this recommendation, so further verification is necessary | |
5.0 |
| ||
| 9.7.1 | 80 D | Partially implemented | |
| PRQA QA-C | Unable to render {include} The included page could not be found. | 3200 | Partially implemented |
Related Vulnerabilities
The vulnerability in Adobe Flash [VU#159523] arises because Flash neglects to check the return value from calloc(). Even when calloc() returns NULL, Flash writes to an offset from the return value. Dereferencing NULL usually results in a program crash, but dereferencing an offset from NULL allows an exploit to succeed without crashing the program.
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
| CERT C Secure Coding Standard | ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy |
| CERT C++ Secure Coding Standard | ERR10-CPP. Check for error conditions FIO04-CPP. Detect and handle input and output errors |
| ISO/IEC TS 17961 | Failing to detect and handle standard library errors [liberr] |
| MITRE CWE | CWE-252, Unchecked return value CWE-253, Incorrect check of function return value CWE-390, Detection of error condition without action CWE-391, Unchecked error condition |
Bibliography
| [DHS 2006] | Handle All Errors Safely |
| [Henricson 1997] | Recommendation 12.1, "Check for All Errors Reported from Functions" |
| [ISO/IEC 9899:2011] | Subclause 7.21.7.10, "The ungetc Function" |