perror
- C course
- Intermediate
- Lesson 54/116
- 5 min
Learn to print error messages with perror.
The perror function
perror writes the errno message to stderr. Usage: perror("cannot open file");. The output combines your prefix, a colon and the system message: cannot open file: No such file or directory.
perror reads errno and prints its text. You don't need strerror to write the message; perror does both. It's the fastest path in debugging because it gives context (prefix) and reason in one line.
stderr and perror
perror writes to stderr, not stdout. This matters: when program output is piped to another program, errors still show on screen. stderr is unbuffered; it appears at once.
perror's format is: your prefix, then a colon, then a space and the errno message. If you don't want a prefix, write perror(""); or perror(NULL);; the system message still prints.
Practice Print a file error with perror
Assign fopen("missing.txt", "r") to FILE *f. If f == NULL, call perror("cannot open file"); and return 1;. Otherwise print printf("open\n"); and close with fclose(f);. End with return 0;.
Starter code
#include <stdio.h>
int main(void) {
// Open missing.txt with fopen
// If NULL, call perror and return 1
// Else printf and fclose
return 0;
}Sample solution
#include <stdio.h>
int main(void) {
FILE *f = fopen("missing.txt", "r");
if (f == NULL) {
perror("cannot open file");
return 1;
}
printf("open\n");
fclose(f);
return 0;
}Expected output
cannot open file: No such file or directory
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the perror call
FILE *f = fopen("missing.txt", "r");
if (f == NULL) ____("cannot open file");FILE *f = fopen("missing.txt", "r");
if (f == NULL) perror("cannot open file");Explanation: perror writes prefix and system message to stderr.
2 Where does perror write its output?
- To stdout
- To a file
- To syslog
- To stderr Correct answer
Explanation: perror writes errors to stderr, so piping doesn't hide them.
3 Match perror output parts
| Term | Meaning |
|---|---|
Prefix |
Your text |
: |
Separator |
System message |
errno meaning |
stderr |
Output stream |