C errno codes: ENOENT, EACCES, EINVAL, ERANGE and what each value means

errno is the integer a C library or system call sets to say why it failed, for example ENOENT (2, no such file or directory) after fopen on a missing path. Read it only after a function has signalled failure, and print it with strerror(errno) or perror(). The names below are portable across POSIX systems; the numbers shown are the Linux values.

Common errno codes (Linux)

Only EDOM, EILSEQ and ERANGE come from ISO C itself. The rest are defined by POSIX, so they exist on Linux and macOS but are not guaranteed by a bare C compiler on every platform.

Print the codes on your system

This program prints the number and the message for several names, so you can check the table against your C library.

mkdir -p /tmp/work && cd /tmp/work
cat > errno_table.c <<'EOF'
#include <errno.h>
#include <stdio.h>
#include <string.h>
#define SHOW(e) printf("%-7s %3d %s\n", #e, e, strerror(e))
int main(void) {
    SHOW(EPERM); SHOW(ENOENT); SHOW(EACCES); SHOW(EEXIST);
    SHOW(EINVAL); SHOW(ENOMEM); SHOW(ERANGE);
    return 0;
}
EOF
gcc -Wall -o errno_table errno_table.c && ./errno_table

EPERM     1 Operation not permitted
ENOENT    2 No such file or directory
EACCES   13 Permission denied
EEXIST   17 File exists
EINVAL   22 Invalid argument
ENOMEM   12 Cannot allocate memory
ERANGE   34 Numerical result out of range

Read errno correctly

1. Check the return value first. fopen returns NULL, open returns -1; only then is errno meaningful.
2. Save it at once. Calls such as printf or fclose can change errno. Write int saved = errno; right after the failing call.
3. Clear it before calls that signal errors only through errno. strtol returns 0 or LONG_MAX on failure and sets ERANGE; set errno = 0 before the call and test it afterwards, as shown in convert a string to int in C.
4. Do not print it when nothing failed. It can hold a stale value from an earlier call.

strerror, perror and thread safety

perror("open") prints open: followed by the message to standard error. strerror(errno) returns the same text as a string, but the buffer may be shared, so use strerror_r in threaded programs. errno itself is thread-local, so each thread sees its own value.

Choosing a response

• EINTR and EAGAIN are usually retried, not reported.
• ENOENT, EACCES and EEXIST are user-facing: show the path and strerror text.
• ENOMEM, EIO and ENOSPC usually mean the operation cannot continue; clean up and exit with a non-zero status.
• EINVAL normally points to a bug in the arguments you passed.

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