C Pocket Reference

Error Handling for System Functions

If an error occurs during a call to a system function, the global error variable errno is assigned an appropriate error code. The following three functions are used to provide the corresponding system error messages:

void perror( const char *string ); Declared in stdio.h

Writes the text pointed to by string, followed by the system error message corresponding to the current value of errno, to the standard error stream.

char *strerror( int errnum ); Declared in string.h

Returns a pointer to the system error message corresponding to errnum. The value of errnum is usually obtained from the error variable errno.

The following two statements result in the same output:

perror( "OPEN" );
fprintf( stderr, "OPEN: %s\n", strerror( errno ) );

void assert( int expression ); Declared in assert.h

This macro tests the scalar expression expression. If the result is 0, or “false”, then assert() writes the expression, function name, filename, and line number to the standard error stream, and then aborts program. If the expression is “true” (i.e., not equal to 0), no action is taken and the program continues.

If the macro NDEBUG is defined, calls to assert() have no effect.