Using errno in C - Step-by-Step Execution

Concept Flow - Using errno

Start program

↓

Call function that may fail

↓

Check if function failed

Yes↓

Read errno value

↓

Use errno to identify error

↓

Handle error or continue

↓

End program

The program calls a function that might fail, then checks errno to find out what error happened, and handles it accordingly.

Execution Sample

C

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

int main() {
    FILE *f = fopen("nofile.txt", "r");
    if (!f) printf("Error: %s\n", strerror(errno));
    return 0;
}

This code tries to open a file that doesn't exist, then prints the error message using errno.

Execution Table

Step	Action	Function Call	errno Value	Output
1	Start program	None	0
2	Call fopen("nofile.txt", "r")	fopen	2
3	Check if fopen returned NULL	NULL check	2
4	Read errno after failure	errno	2
5	Print error message	strerror(errno)	2	Error: No such file or directory
6	End program	None	2

💡 fopen fails because file does not exist, errno is set to 2 (ENOENT)

Variable Tracker

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

Key Moments - 3 Insights

Why do we check if fopen returned NULL before reading errno?

Because errno is only meaningful if the function failed. Checking fopen's return (row 3) ensures we only read errno when an error happened.

Does errno reset automatically before each function call?

No, errno keeps its value until changed by a failing function. It should be set to 0 manually if you want to detect new errors reliably.

Why do we use strerror(errno) to print the error?

Because errno is just a number. strerror converts that number into a readable message (row 5 shows this).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of errno right after fopen fails?

A 2

B 0

C 1

D Undefined

💡 Hint

Check Step 2 and Step 4 in the execution_table where errno is set after fopen fails.

At which step does the program print the error message?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the Output column in the execution_table to find when the error message appears.

If fopen succeeded, what would be the value of errno at Step 4?

A 2

B It would be unchanged from before

C 0

D Some error code

💡 Hint

Refer to key_moments about errno not resetting automatically and only changing on failure.

Concept Snapshot

Using errno in C:
- Include <errno.h> and <string.h>
- Call a function that may fail (e.g., fopen)
- Check if function failed (e.g., fopen returns NULL)
- Read errno to get error code
- Use strerror(errno) to get readable message
- errno is set only on failure, not reset automatically

Full Transcript

This example shows how to use errno in C. The program tries to open a file that does not exist. fopen returns NULL to indicate failure. errno is set to 2, which means 'No such file or directory'. We check if fopen failed before reading errno. Then we use strerror(errno) to print a friendly error message. errno keeps its value until changed by another failing function. It does not reset automatically. This helps us understand how to detect and handle errors in C programs.