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In general, you should detect errors by checking return values, and use errno only to distinguish among the various causes of an error, such as ``File not found'' or ``Permission denied''. (Typically, you use perror or strerror to print these discriminating error messages.) It's only necessary to detect errors with errno when a function does not have a unique, unambiguous, out-of-band error return (i.e. because all of its possible return values are valid; one example is atoi). In these cases (and in these cases only; check the documentation to be sure whether a function allows this), you can detect errors by setting errno to 0, calling the function, then testing errno. (Setting errno to 0 first is important, as no library function ever does that for you.)

To make error messages useful, they should include all relevant information. Besides the strerror text derived from errno, it may also be appropriate to print the name of the program, the operation which failed (preferably in terms which will be meaningful to the user), the name of the file for which the operation failed, and, if some input file (script or source file) is being read, the name and current line number of that file.

Non-Compliant Code Example (Memory Management)

This example, taken from \[[MEM32-C. Detect and handle critical memory allocation errors]\] demonstrates why checking the return value of memory allocation routines is critical. The buffer {{input_string}} is copied into dynamically allocated memory referenced by {{str}}. However, the result of {{malloc()}} is not checked before {{str}} is referenced. Consequently, if {{malloc()}} fails, the program will abnormally terminate. 

/* ... */
size_t size = strlen(input_string);
if (size == SIZE_MAX) {
  /* Handle Error */
}
str = malloc(size+1);
strcpy(str, input_string);
/* ... */
free(str);

Compliant Solution

Upon failure, the malloc() function returns NULL. Failing to handle this error condition appropriately can lead to abnormal and abrupt program termination.

/* ... */
size_t size = strlen(input_string);
if (size == SIZE_MAX) {
  /* Handle Error */
}
str = malloc(size+1);
if (str == NULL) {
  /* Handle Allocation Error */
}
strcpy(str, input_string);
/* ... */
free(str);

References

Failing to detect error condition can result in unexpected program behavior, and possibly abnormal program termination resulting in a denial-of-service condition.

C-FAQ Question 20.4
References: ISO Sec. 7.1.4, Sec. 7.9.10.4, Sec. 7.11.6.2
CT&P Sec. 5.4 p. 73
PCS Sec. 11 p. 168, Sec. 14 p. 254