The perror() function produces a message on standard error describing the last error encountered during a call to a system or library function.
When a system call fails, it usually returns -1 and sets the variable errno to a value describing what went wrong. (These values can be found in .) Many library functions do likewise. The function perror() serves to translate this error code into human-readable form.
If we write a simple program to demonstrate this, it could be as following,
#include <stdio.h>
#include <errno.h>
int main(int argc, char **argv) {
FILE *fp = NULL;
fp = fopen("this_is_perror_test.txt", "r");
if (fp == NULL) {
perror("fopen");
}
return 0;
}
OR this same program can also be written using strerror as,
#include <stdio.h>
#include <errno.h>
#include <string.h>
int main(int argc, char **argv) {
FILE *fp = NULL;
fp = fopen("this_is_perror_test.txt", "r");
if (fp == NULL) {
fprintf(stderr, "fopen: %s\n", strerror(errno));
}
return 0;
}
$ gcc -o simple_perror simple_perror.c
If we run the program now as,
$./simple_perror fopen: No such file or directory
Above program show, how an error number of ENOENT gets automatically assigned as error code and perror prints human readable message as “No such file or directory”
Below program shows some of the other probable error codes and messages as,
#include <stdio.h>
#include <errno.h>
int main(int argc, char **argv) {
int i=0;
for (i=1; i<=133; i++) {
errno = i;
switch (i) {
case EPERM: perror("EPERM"); break;
case ENOENT: perror("ENOENT"); break;
case ESRCH: perror("ESRCH"); break;
case EINTR: perror("EINTR"); break;
case EIO: perror("EIO"); break;
case ENXIO: perror("ENXIO"); break;
case E2BIG: perror("E2BIG"); break;
case ENOEXEC: perror("ENOEXEC"); break;
case EBADF: perror("EBADF"); break;
case ECHILD: perror("ECHILD"); break;
case EAGAIN: perror("EAGAIN"); break;
case ENOMEM: perror("ENOMEM"); break;
case EACCES: perror("EACCES"); break;
case EFAULT: perror("EFAULT"); break;
case ENOTBLK: perror("ENOTBLK"); break;
case EBUSY: perror("EBUSY"); break;
case EEXIST: perror("EEXIST"); break;
case EXDEV: perror("EXDEV"); break;
case ENODEV: perror("ENODEV"); break;
case ENOTDIR: perror("ENOTDIR"); break;
case EISDIR: perror("EISDIR"); break;
case EINVAL: perror("EINVAL"); break;
case ENFILE: perror("ENFILE"); break;
case EMFILE: perror("EMFILE"); break;
case ENOTTY: perror("ENOTTY"); break;
case ETXTBSY: perror("ETXTBSY"); break;
case EFBIG: perror("EFBIG"); break;
case ENOSPC: perror("ENOSPC"); break;
case ESPIPE: perror("ESPIPE"); break;
case EROFS: perror("EROFS"); break;
case EMLINK: perror("EMLINK"); break;
case EPIPE: perror("EPIPE"); break;
case EDOM: perror("EDOM"); break;
case ERANGE: perror("ERANGE"); break;
case EDEADLK: perror("EDEADLK"); break;
case ENAMETOOLONG: perror("ENAMETOOLONG"); break;
case ENOLCK: perror("ENOLCK"); break;
case ENOSYS: perror("ENOSYS"); break;
case ENOTEMPTY: perror("ENOTEMPTY"); break;
case ELOOP: perror("ELOOP"); break;
case ENOMSG: perror("ENOMSG"); break;
case EIDRM: perror("EIDRM"); break;
case ECHRNG: perror("ECHRNG"); break;
case EL2NSYNC: perror("EL2NSYNC"); break;
case EL3HLT: perror("EL3HLT"); break;
case EL3RST: perror("EL3RST"); break;
case ELNRNG: perror("ELNRNG"); break;
case EUNATCH: perror("EUNATCH"); break;
case ENOCSI: perror("ENOCSI"); break;
case EL2HLT: perror("EL2HLT"); break;
case EBADE: perror("EBADE"); break;
case EBADR: perror("EBADR"); break;
case EXFULL: perror("EXFULL"); break;
case ENOANO: perror("ENOANO"); break;
case EBADRQC: perror("EBADRQC"); break;
case EBADSLT: perror("EBADSLT"); break;
case EBFONT: perror("EBFONT"); break;
case ENOSTR: perror("ENOSTR"); break;
case ENODATA: perror("ENODATA"); break;
case ETIME: perror("ETIME"); break;
case ENOSR: perror("ENOSR"); break;
case ENONET: perror("ENONET"); break;
case ENOPKG: perror("ENOPKG"); break;
case EREMOTE: perror("EREMOTE"); break;
case ENOLINK: perror("ENOLINK"); break;
case EADV: perror("EADV"); break;
case ESRMNT: perror("ESRMNT"); break;
case ECOMM: perror("ECOMM"); break;
case EPROTO: perror("EPROTO"); break;
case EMULTIHOP: perror("EMULTIHOP"); break;
case EDOTDOT: perror("EDOTDOT"); break;
case EBADMSG: perror("EBADMSG"); break;
case EOVERFLOW: perror("EOVERFLOW"); break;
case ENOTUNIQ: perror("ENOTUNIQ"); break;
case EBADFD: perror("EBADFD"); break;
case EREMCHG: perror("EREMCHG"); break;
case ELIBACC: perror("ELIBACC"); break;
case ELIBBAD: perror("ELIBBAD"); break;
case ELIBSCN: perror("ELIBSCN"); break;
case ELIBMAX: perror("ELIBMAX"); break;
case ELIBEXEC: perror("ELIBEXEC"); break;
case EILSEQ: perror("EILSEQ"); break;
case ERESTART: perror("ERESTART"); break;
case ESTRPIPE: perror("ESTRPIPE"); break;
case EUSERS: perror("EUSERS"); break;
case ENOTSOCK: perror("ENOTSOCK"); break;
case EDESTADDRREQ: perror("EDESTADDRREQ"); break;
case EMSGSIZE: perror("EMSGSIZE"); break;
case EPROTOTYPE: perror("EPROTOTYPE"); break;
case ENOPROTOOPT: perror("ENOPROTOOPT"); break;
case EPROTONOSUPPORT: perror("EPROTONOSUPPORT"); break;
case ESOCKTNOSUPPORT: perror("ESOCKTNOSUPPORT"); break;
case EOPNOTSUPP: perror("EOPNOTSUPP"); break;
case EPFNOSUPPORT: perror("EPFNOSUPPORT"); break;
case EAFNOSUPPORT: perror("EAFNOSUPPORT"); break;
case EADDRINUSE: perror("EADDRINUSE"); break;
case EADDRNOTAVAIL: perror("EADDRNOTAVAIL"); break;
case ENETDOWN: perror("ENETDOWN"); break;
case ENETUNREACH: perror("ENETUNREACH"); break;
case ENETRESET: perror("ENETRESET"); break;
case ECONNABORTED: perror("ECONNABORTED"); break;
case ECONNRESET: perror("ECONNRESET"); break;
case ENOBUFS: perror("ENOBUFS"); break;
case EISCONN: perror("EISCONN"); break;
case ENOTCONN: perror("ENOTCONN"); break;
case ESHUTDOWN: perror("ESHUTDOWN"); break;
case ETOOMANYREFS: perror("ETOOMANYREFS"); break;
case ETIMEDOUT: perror("ETIMEDOUT"); break;
case ECONNREFUSED: perror("ECONNREFUSED"); break;
case EHOSTDOWN: perror("EHOSTDOWN"); break;
case EHOSTUNREACH: perror("EHOSTUNREACH"); break;
case EALREADY: perror("EALREADY"); break;
case EINPROGRESS: perror("EINPROGRESS"); break;
case ESTALE: perror("ESTALE"); break;
case EUCLEAN: perror("EUCLEAN"); break;
case ENOTNAM: perror("ENOTNAM"); break;
case ENAVAIL: perror("ENAVAIL"); break;
case EISNAM: perror("EISNAM"); break;
case EREMOTEIO: perror("EREMOTEIO"); break;
case EDQUOT: perror("EDQUOT"); break;
case ENOMEDIUM: perror("ENOMEDIUM"); break;
case EMEDIUMTYPE: perror("EMEDIUMTYPE"); break;
case ECANCELED: perror("ECANCELED"); break;
case ENOKEY: perror("ENOKEY"); break;
case EKEYEXPIRED: perror("EKEYEXPIRED"); break;
case EKEYREVOKED: perror("EKEYREVOKED"); break;
case EKEYREJECTED: perror("EKEYREJECTED"); break;
case EOWNERDEAD: perror("EOWNERDEAD"); break;
case ENOTRECOVERABLE: perror("ENOTRECOVERABLE"); break;
case ERFKILL: perror("ERFKILL"); break;
case EHWPOISON: perror("EHWPOISON"); break;
}
}
return 0;
}
compile the program as,
$ gcc -o perror_errno_description perror_errno_description.c
$ ./perror_errno_description
The output of above execution will display the error number and its related meaningful description as,
EPERM: Operation not permitted ENOENT: No such file or directory ESRCH: No such process EINTR: Interrupted system call EIO: Input/output error ENXIO: No such device or address E2BIG: Argument list too long ENOEXEC: Exec format error EBADF: Bad file descriptor ECHILD: No child processes EAGAIN: Resource temporarily unavailable ENOMEM: Cannot allocate memory EACCES: Permission denied EFAULT: Bad address ENOTBLK: Block device required EBUSY: Device or resource busy EEXIST: File exists EXDEV: Invalid cross-device link ENODEV: No such device ENOTDIR: Not a directory EISDIR: Is a directory EINVAL: Invalid argument ENFILE: Too many open files in system EMFILE: Too many open files ENOTTY: Inappropriate ioctl for device ETXTBSY: Text file busy EFBIG: File too large ENOSPC: No space left on device ESPIPE: Illegal seek EROFS: Read-only file system EMLINK: Too many links EPIPE: Broken pipe EDOM: Numerical argument out of domain ERANGE: Numerical result out of range EDEADLK: Resource deadlock avoided ENAMETOOLONG: File name too long ENOLCK: No locks available ENOSYS: Function not implemented ENOTEMPTY: Directory not empty ELOOP: Too many levels of symbolic links ENOMSG: No message of desired type EIDRM: Identifier removed ECHRNG: Channel number out of range EL2NSYNC: Level 2 not synchronized EL3HLT: Level 3 halted EL3RST: Level 3 reset ELNRNG: Link number out of range EUNATCH: Protocol driver not attached ENOCSI: No CSI structure available EL2HLT: Level 2 halted EBADE: Invalid exchange EBADR: Invalid request descriptor EXFULL: Exchange full ENOANO: No anode EBADRQC: Invalid request code EBADSLT: Invalid slot EBFONT: Bad font file format ENOSTR: Device not a stream ENODATA: No data available ETIME: Timer expired ENOSR: Out of streams resources ENONET: Machine is not on the network ENOPKG: Package not installed EREMOTE: Object is remote ENOLINK: Link has been severed EADV: Advertise error ESRMNT: Srmount error ECOMM: Communication error on send EPROTO: Protocol error EMULTIHOP: Multihop attempted EDOTDOT: RFS specific error EBADMSG: Bad message EOVERFLOW: Value too large for defined data type ENOTUNIQ: Name not unique on network EBADFD: File descriptor in bad state EREMCHG: Remote address changed ELIBACC: Can not access a needed shared library ELIBBAD: Accessing a corrupted shared library ELIBSCN: .lib section in a.out corrupted ELIBMAX: Attempting to link in too many shared libraries ELIBEXEC: Cannot exec a shared library directly EILSEQ: Invalid or incomplete multibyte or wide character ERESTART: Interrupted system call should be restarted ESTRPIPE: Streams pipe error EUSERS: Too many users ENOTSOCK: Socket operation on non-socket EDESTADDRREQ: Destination address required EMSGSIZE: Message too long EPROTOTYPE: Protocol wrong type for socket ENOPROTOOPT: Protocol not available EPROTONOSUPPORT: Protocol not supported ESOCKTNOSUPPORT: Socket type not supported EOPNOTSUPP: Operation not supported EPFNOSUPPORT: Protocol family not supported EAFNOSUPPORT: Address family not supported by protocol EADDRINUSE: Address already in use EADDRNOTAVAIL: Cannot assign requested address ENETDOWN: Network is down ENETUNREACH: Network is unreachable ENETRESET: Network dropped connection on reset ECONNABORTED: Software caused connection abort ECONNRESET: Connection reset by peer ENOBUFS: No buffer space available EISCONN: Transport endpoint is already connected ENOTCONN: Transport endpoint is not connected ESHUTDOWN: Cannot send after transport endpoint shutdown ETOOMANYREFS: Too many references: cannot splice ETIMEDOUT: Connection timed out ECONNREFUSED: Connection refused EHOSTDOWN: Host is down EHOSTUNREACH: No route to host EALREADY: Operation already in progress EINPROGRESS: Operation now in progress ESTALE: Stale file handle EUCLEAN: Structure needs cleaning ENOTNAM: Not a XENIX named type file ENAVAIL: No XENIX semaphores available EISNAM: Is a named type file EREMOTEIO: Remote I/O error EDQUOT: Disk quota exceeded ENOMEDIUM: No medium found EMEDIUMTYPE: Wrong medium type ECANCELED: Operation canceled ENOKEY: Required key not available EKEYEXPIRED: Key has expired EKEYREVOKED: Key has been revoked EKEYREJECTED: Key was rejected by service EOWNERDEAD: Owner died ENOTRECOVERABLE: State not recoverable ERFKILL: Operation not possible due to RF-kill EHWPOISON: Memory page has hardware error