Page 156 - C-Language

Chapter 21: Error handling

Syntax

• #include <errno.h>
• int errno; /* implementation defined */
• #include <string.h>
• char *strerror(int errnum);
• #include <stdio.h>
• void perror(const char *s);

Remarks

Have in mind that errno is not necessarily a variable but that the syntax is only an indication how it
might been declared. On many modern systems with thread interfaces errno is some macro that
resolves to an object that is local to the current thread.

Examples

errno

When a standard library function fails, it often sets errno to the appropriate error code. The C
standard requires at least 3 values for errno be set:

Value Meaning

EDOM Domain error

ERANGE Range error

EILSEQ Illegal multi-byte character sequence

strerror

If perror is not flexible enough, you may obtain a user-readable error description by calling
strerror from <string.h>.

int main(int argc, char *argv[])
{
FILE *fout;
int last_error = 0;

if ((fout = fopen(argv[1], "w")) == NULL) {
last_error = errno;
/* reset errno and continue */
errno = 0;

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