Lesson 55: Error Handling: errno and perror

In C, many standard library functions (especially I/O and system calls) signal failure by returning a special value (like NULL for fopen or -1 for system calls). They simultaneously set a global integer variable, errno, to indicate why the failure occurred.

The errno Variable

• Defined in <errno.h>.
• It is set by system calls and library functions upon failure.
• Its value is a coded integer representing the specific error type (e.g., 'File not found', 'Permission denied').

Crucial: errno is only meaningful immediately after a function fails. Its value is not guaranteed to be preserved across unrelated successful calls.

The perror() Function

perror() prints a descriptive error message to stderr (Standard Error) based on the current value of errno. It allows you to prefix the error message with a custom string.

Syntax: void perror(const char *s);

Example: File Open Error

c #include <stdio.h> #include <errno.h>

int main() { FILE *fp;

// Trying to open a non-existent file for reading
fp = fopen("nonexistent.dat", "r");

if (fp == NULL) {
    // Use perror to print a descriptive error message
    perror("File Operation Failed");
    // The output might look like: 
    // File Operation Failed: No such file or directory
    return 1;
}

fclose(fp);
return 0;

}

strerror()

The strerror() function (in <string.h>) returns a pointer to the human-readable string corresponding to a given error number, which is useful if you want to log the error message instead of immediately printing it.