When calling execl(...), I get an errno=2. What does it mean? How can I know the meaning of this errno?
15 Answers
You can use strerror() to get a human-readable string for the error number. This is the same string printed by perror() but it's useful if you're formatting the error message for something other than standard error output.
For example:
#include <errno.h> #include <string.h> /* ... */ if(read(fd, buf, 1)==-1) { printf("Oh dear, something went wrong with read()! %s\n", strerror(errno)); } Linux also supports the explicitly-threadsafe variant strerror_r().
Instead of running perror on any error code you get, you can retrieve a complete listing of errno values on your system with the following one-liner:
cpp -dM /usr/include/errno.h | grep 'define E' | sort -n -k 3
On Linux there is also a very neat tool that can tell right away what each error code means. On Ubuntu: apt-get install errno.
Then if for example you want to get the description of error type 2, just type errno 2 in the terminal.
With errno -l you get a list with all errors and their descriptions. Much easier that other methods mentioned by previous posters.
Here is the output from errno -l reformatted for readability:
1 EPERM Operation not permitted 2 ENOENT No such file or directory 3 ESRCH No such process 4 EINTR Interrupted system call 5 EIO Input/output error 6 ENXIO No such device or address 7 E2BIG Argument list too long 8 ENOEXEC Exec format error 9 EBADF Bad file descriptor 10 ECHILD No child processes 11 EAGAIN Resource temporarily unavailable 11 EWOULDBLOCK Resource temporarily unavailable 12 ENOMEM Cannot allocate memory 13 EACCES Permission denied 14 EFAULT Bad address 15 ENOTBLK Block device required 16 EBUSY Device or resource busy 17 EEXIST File exists 18 EXDEV Invalid cross-device link 19 ENODEV No such device 20 ENOTDIR Not a directory 21 EISDIR Is a directory 22 EINVAL Invalid argument 23 ENFILE Too many open files in system 24 EMFILE Too many open files 25 ENOTTY Inappropriate ioctl for device 26 ETXTBSY Text file busy 27 EFBIG File too large 28 ENOSPC No space left on device 29 ESPIPE Illegal seek 30 EROFS Read-only file system 31 EMLINK Too many links 32 EPIPE Broken pipe 33 EDOM Numerical argument out of domain 34 ERANGE Numerical result out of range 35 EDEADLK Resource deadlock avoided 35 EDEADLOCK Resource deadlock avoided 36 ENAMETOOLONG File name too long 37 ENOLCK No locks available 38 ENOSYS Function not implemented 39 ENOTEMPTY Directory not empty 40 ELOOP Too many levels of symbolic links 42 ENOMSG No message of desired type 43 EIDRM Identifier removed 44 ECHRNG Channel number out of range 45 EL2NSYNC Level 2 not synchronized 46 EL3HLT Level 3 halted 47 EL3RST Level 3 reset 48 ELNRNG Link number out of range 49 EUNATCH Protocol driver not attached 50 ENOCSI No CSI structure available 51 EL2HLT Level 2 halted 52 EBADE Invalid exchange 53 EBADR Invalid request descriptor 54 EXFULL Exchange full 55 ENOANO No anode 56 EBADRQC Invalid request code 57 EBADSLT Invalid slot 59 EBFONT Bad font file format 60 ENOSTR Device not a stream 61 ENODATA No data available 62 ETIME Timer expired 63 ENOSR Out of streams resources 64 ENONET Machine is not on the network 65 ENOPKG Package not installed 66 EREMOTE Object is remote 67 ENOLINK Link has been severed 68 EADV Advertise error 69 ESRMNT Srmount error 70 ECOMM Communication error on send 71 EPROTO Protocol error 72 EMULTIHOP Multihop attempted 73 EDOTDOT RFS specific error 74 EBADMSG Bad message 75 EOVERFLOW Value too large for defined data type 76 ENOTUNIQ Name not unique on network 77 EBADFD File descriptor in bad state 78 EREMCHG Remote address changed 79 ELIBACC Can not access a needed shared library 80 ELIBBAD Accessing a corrupted shared library 81 ELIBSCN .lib section in a.out corrupted 82 ELIBMAX Attempting to link in too many shared libraries 83 ELIBEXEC Cannot exec a shared library directly 84 EILSEQ Invalid or incomplete multibyte or wide character 85 ERESTART Interrupted system call should be restarted 86 ESTRPIPE Streams pipe error 87 EUSERS Too many users 88 ENOTSOCK Socket operation on non-socket 89 EDESTADDRREQ Destination address required 90 EMSGSIZE Message too long 91 EPROTOTYPE Protocol wrong type for socket 92 ENOPROTOOPT Protocol not available 93 EPROTONOSUPPORT Protocol not supported 94 ESOCKTNOSUPPORT Socket type not supported 95 ENOTSUP Operation not supported 95 EOPNOTSUPP Operation not supported 96 EPFNOSUPPORT Protocol family not supported 97 EAFNOSUPPORT Address family not supported by protocol 98 EADDRINUSE Address already in use 99 EADDRNOTAVAIL Cannot assign requested address 100 ENETDOWN Network is down 101 ENETUNREACH Network is unreachable 102 ENETRESET Network dropped connection on reset 103 ECONNABORTED Software caused connection abort 104 ECONNRESET Connection reset by peer 105 ENOBUFS No buffer space available 106 EISCONN Transport endpoint is already connected 107 ENOTCONN Transport endpoint is not connected 108 ESHUTDOWN Cannot send after transport endpoint shutdown 109 ETOOMANYREFS Too many references: cannot splice 110 ETIMEDOUT Connection timed out 111 ECONNREFUSED Connection refused 112 EHOSTDOWN Host is down 113 EHOSTUNREACH No route to host 114 EALREADY Operation already in progress 115 EINPROGRESS Operation now in progress 116 ESTALE Stale file handle 117 EUCLEAN Structure needs cleaning 118 ENOTNAM Not a XENIX named type file 119 ENAVAIL No XENIX semaphores available 120 EISNAM Is a named type file 121 EREMOTEIO Remote I/O error 122 EDQUOT Disk quota exceeded 123 ENOMEDIUM No medium found 124 EMEDIUMTYPE Wrong medium type 125 ECANCELED Operation canceled 126 ENOKEY Required key not available 127 EKEYEXPIRED Key has expired 128 EKEYREVOKED Key has been revoked 129 EKEYREJECTED Key was rejected by service 130 EOWNERDEAD Owner died 131 ENOTRECOVERABLE State not recoverable 132 ERFKILL Operation not possible due to RF-kill 133 EHWPOISON Memory page has hardware error I used tabularise in Vim to align the columns:
:%Tab /^[^ ]*\zs /r1l1l1 :%Tab /^ *[^ ]* *[^ ]*\zs /l1 Error code 2 means "File/Directory not found". In general, you could use the perror function to print a human readable string.
There's a few useful functions for dealing with errnos. (Just to make it clear, these are built-in to libc -- I'm just providing sample implementations because some people find reading code clearer than reading English.)
#include <string.h> char *strerror(int errnum); /* you can think of it as being implemented like this: */ static char strerror_buf[1024]; const char *sys_errlist[] = { [EPERM] = "Operation not permitted", [ENOENT] = "No such file or directory", [ESRCH] = "No such process", [EINTR] = "Interrupted system call", [EIO] = "I/O error", [ENXIO] = "No such device or address", [E2BIG] = "Argument list too long", /* etc. */ }; int sys_nerr = sizeof(sys_errlist) / sizeof(char *); char *strerror(int errnum) { if (0 <= errnum && errnum < sys_nerr && sys_errlist[errnum]) strcpy(strerror_buf, sys_errlist[errnum]); else sprintf(strerror_buf, "Unknown error %d", errnum); return strerror_buf; } strerror returns a string describing the error number you've passed to it. Caution, this is not thread- or interrupt-safe; it is free to rewrite the string and return the same pointer on the next invocation. Use strerror_r if you need to worry about that.
#include <stdio.h> void perror(const char *s); /* you can think of it as being implemented like this: */ void perror(const char *s) { fprintf(stderr, "%s: %s\n", s, strerror(errno)); } perror prints out the message you give it, plus a string describing the current errno, to standard error.
This is faster than looking up the code in errno.h, shorter than most solutions posted here and it does not require installation of third party tools:
perl -E 'say $!=shift' 2
yields
No such file or directory
#include <errno.h> #include <stdio.h> #include <stdlib.h> int main(int i, char *c[]) { if (i != 2) fprintf(stderr, "Usage: perror errno\n"); else { errno = atoi(c[1]); perror(""); } exit(0); } Works on Solaris.
cc perror.c -o perror << use this line to compile it
Here is the documentation. That should tell you what it means and what to do with them. You should avoid using the numeric value and use the constants listed there as well, as the number may change between different systems.
4I use the following script:
#!/usr/bin/python import errno import os import sys toname = dict((str(getattr(errno, x)), x) for x in dir(errno) if x.startswith("E")) tocode = dict((x, getattr(errno, x)) for x in dir(errno) if x.startswith("E")) for arg in sys.argv[1:]: if arg in tocode: print arg, tocode[arg], os.strerror(tocode[arg]) elif arg in toname: print toname[arg], arg, os.strerror(int(arg)) else: print "Unknown:", arg Type sudo apt-get install moreutils into your shell and then, once that has installed, type errno 2. You can also use errno -l for all error numbers, or see only the file ones by piping it to grep, like this: errno -l | grep file.
Call
perror("execl"); in case of error.
Sample:
if(read(fd, buf, 1)==-1) { perror("read"); } The manpages of errno(3) and perror(3) are interesting, too...
When you use strace (on Linux) to run your binary, it will output the returns from system calls and what the error number means. This may sometimes be useful to you.
I have the following function in my .bashrc file - it looks up the errno value from the header files (can be either /usr/include/errno.h, /usr/include/linux/errno.h, etc., etc.)
It works if header files are installed on the machine;-)
Usually the header file have an error + next comes the explanation in the comment; something of the following:
./asm-generic/errno-base.h:#define EAGAIN 11 /* Try again */
function errno() { local arg=$1 if [[ "x$arg" == "x-h" ]]; then cat <<EOF Usage: errno <num> Prints text that describes errno error number EOF else pushd /usr/include find . -name "errno*.h" | xargs grep "[[:space:]]${arg}[[:space:]]" popd fi } It means:
File or Directory not found.
