perror and strerror functions - Step-by-Step Execution

Concept Flow - perror and strerror functions

Error Occurs

↓

Call perror(msg)

→Prints: msg + error description

↓

OR

↓

Call strerror(errno)

→Returns error description string

↓

Use returned string for custom output

When an error happens, perror prints a message plus the error text. strerror returns the error text as a string to use in your code.

Execution Sample

C

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

int main() {
  FILE *f = fopen("nofile.txt", "r");
  if (!f) perror("File open error");
  return 0;
}

This code tries to open a missing file and uses perror to print the error message.

Execution Table

Step	Action	errno Value	Function Called	Output
1	Try to open 'nofile.txt' for reading	2 (ENOENT)	fopen	NULL returned (file not found)
2	Check if fopen returned NULL	2	if condition	True, enter error handling
3	Call perror("File open error")	2	perror	File open error: No such file or directory
4	Program ends	2	return 0	No output

💡 fopen fails, errno set to 2 (file not found), perror prints message, program ends

Variable Tracker

Variable	Start	After fopen	After perror	Final
f	undefined	NULL	NULL	NULL
errno	0	2	2	2

Key Moments - 3 Insights

Why does perror print both my message and the error description?

perror automatically appends the system error message for the current errno after your message, as shown in step 3 of the execution_table.

What does strerror(errno) return and how is it different from perror?

strerror(errno) returns a string describing the error but does not print it. You can use it to build custom messages, unlike perror which prints directly (see concept_flow).

Why is errno set after fopen fails?

When fopen fails, it sets errno to indicate the error cause (like file not found). This is why perror and strerror can show the correct message (see step 1 and 2 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of errno after fopen fails?

A 1

B 2

C 0

D Undefined

💡 Hint

Check step 1 and 2 in the execution_table where errno is shown after fopen fails.

At which step does perror print the error message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Function Called' and 'Output' columns in the execution_table.

If we replace perror with printf("Error: %s\n", strerror(errno)), what changes in output?

A Output will miss the custom message prefix

B Output will be the same

C Output will include extra system info

D No output will be printed

💡 Hint

Refer to the concept_flow and key_moments about how perror and strerror differ in output.

Concept Snapshot

perror(msg): prints msg + system error text for errno
strerror(errno): returns error text string for errno
Use perror for quick error print
Use strerror for custom error handling
errno holds last error code set by system calls

Full Transcript

When a system call or library function fails, it sets a global variable called errno to a number representing the error. The perror function prints a message you give it, followed by a colon and the system's description of that error number. The strerror function returns the error description as a string, so you can use it in your own messages. In the example, fopen tries to open a file that does not exist, so it returns NULL and sets errno to 2, meaning 'No such file or directory'. Then perror prints 'File open error: No such file or directory'. This helps you understand what went wrong quickly. strerror can be used if you want to build your own error messages instead of printing directly.