Example of realpath function in C

Question

I'm looking for an example of how to use the realpath function in a C program. I can't seem to find one on the web or in any of my C programming books.

Jana

Answer 1

• Note

  • The realpath() function is not described in the C Standard
    • It is however described by POSIX 1997 and POSIX 2008.

• Sample Code

#include <limits.h> /* PATH_MAX */
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    char buf[PATH_MAX]; /* PATH_MAX incudes the \0 so +1 is not required */
    char *res = realpath("this_source.c", buf);
    if (res) { // or: if (res != NULL)
        printf("This source is at %s.\n", buf);
    } else {
        char* errStr = strerror(errno);
        printf("error string: %s\n", errStr);

        perror("realpath");
        exit(EXIT_FAILURE);
    }
    return 0;
}

• Reference
  • PATH_MAX defined in PATH_MAX from POSIX 1997
  • errno
  • strerror

Acidimetry

PATH_MAX is the maximum length for a path. It is a weird gizmo; there is guaranteed to be a value _POSIX_PATH_MAX which is the minimum value that PATH_MAX may be. However, many systems have a <limits.h> that does not set PATH_MAX, meaning there isn't a prescribed limit for the maximum length of a path on the machine. It can also be file-system dependent, anyway. So, you can look up a value with sysconf(), or with pathconf(), or you can take a guess that most sane people don't use paths longer than 1024 bytes and use that. – Indictable

you should take into consideration calling free(res); after calling realpath and finishing from res. As realpath allocates the return value in the heap. Source: man realpath – Lynnalynne

@JalalMostafa: POSIX realpath(), as described in the link in my answer, does not allocate memory. I suspect your system is not POSIX compliant. Calling POSIX compliant realpath() with NULL for the 2nd argument invokes UB. – Acidimetry

@JalalMostafa: There is a new version of the POSIX specification (realpath() in POSIX 2008) there the 2nd argument can be NULL and then malloc() is called. In my example, the buffer is a local array, so no cause for calling free(). Thanks for the heads up. – Acidimetry

Answer 2

One-line build command line

Minimalist but it does the job!

Build

gcc -o realpath -x c - <<< $'#include<stdlib.h>\n#include<stdio.h>\nint main(int c,char**v){puts(realpath(v[1],0));}'

Test

$> ./realpath  ~/../../../usr/./bin/./awk
/bin/gawk 

$> readlink -f ~/../../../usr/./bin/./awk
/bin/gawk

Requirements

• gcc for compilation and link
• bash for <<< and $' ... \n ... '

Crash

My minimalist one-line command line builds an executable realpath that produces a Segmentation fault when the path does not exist. Instead of writing if/else blocs to handle that issue within my answer, I have added below some links to let you have a look on the Busybox implementation of realpath and readlink.

---

Busybox implementation

For a more complete source code, have a look on this simple implementation.

Official Git repository

• coreutils/realpath.c
• coreutils/readlink.c
• libbb/xreadlink.c

GitHub mirror repository

• coreutils/realpath.c
• coreutils/readlink.c
• libbb/xreadlink.c

Mehta

/usr/bin/readlink -f does not require the path exist. However, realpath api does. Could you help to comment how to mimic readlink in this respect ? – Nicknickel

Hi @SOUser. Thank you for your feedback. You are right, my minimalist one-line command line builds an executable realpath that produces a Segmentation fault when path does not exist. Instead of writing if/else blocs to handle that within my answer, I have added links to let you have a look on the Busybox implementation of realpath and readlink. I hope you appreciate the Busybox source code. Please let me know if I can help. Cheers – Mehta

Answer 3

What the realpath() function does is tell you the absolute pathname of a file when all symbolic links have been resolved.

You may have a 'realpath' program already on your machine. Here's the (non-standard) version I wrote.

/*
@(#)File:           $RCSfile: realpath.c,v $
@(#)Version:        $Revision: 1.3 $
@(#)Last changed:   $Date: 2007/10/23 20:23:44 $
@(#)Purpose:        Command to evaluate realpath(3) on given arguments.
@(#)Author:         J Leffler
@(#)Copyright:      (C) JLSS 2007
@(#)Product:        :PRODUCT:
*/

/*TABSTOP=4*/

#if __STDC_VERSION__ >= 199901L
#define _XOPEN_SOURCE 600
#else
#define _XOPEN_SOURCE 500
#endif /* __STDC_VERSION__ */

#include <unistd.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include "stderr.h"

static const char optstr[] = "hlsV";
static const char usestr[] = "[-hslV] given-path [...]";
static const char hlpstr[] =
    "  -h   Print this help message\n"
    "  -l   Long format: print given-path and real-path\n"
    "  -s   Short format: print just real-path\n"
    "  -V   Print version and exit\n"
    ;

enum { FMT_LONG, FMT_SHORT };
static int format_type = FMT_LONG;

#ifndef lint
/* Prevent over-aggressive optimizers from eliminating ID string */
extern const char jlss_id_realpath_c[];
const char jlss_id_realpath_c[] = "@(#)$Id: realpath.c,v 1.3 2007/10/23 20:23:44 jleffler Exp $";
#endif /* lint */

static int eval_realpath(const char *given)
{
    char realname[_POSIX_PATH_MAX];
    int rc = 0;

    if (realpath(given, realname) == 0)
    {
        rc = -1;
        err_sysrem("failed to resolve real path name for %s\n", given);
    }
    else if (format_type == FMT_SHORT)
        printf("%s\n", realname);
    else
        printf("%s %s\n", given, realname);
    return(rc);
}

int main(int argc, char **argv)
{
    int i;
    int rc = EXIT_SUCCESS;
    int opt;

    err_setarg0(argv[0]);
    while ((opt = getopt(argc, argv, optstr)) != -1)
    {
        switch (opt)
        {
        case 'V':
            err_version("REALPATH", &"@(#)$Revision: 1.3 $ ($Date: 2007/10/23 20:23:44 $)"[4]);
            break;
        case 'h':
            err_help(usestr, hlpstr);
            break;
        case 'l':
            format_type = FMT_LONG;
            break;
        case 's':
            format_type = FMT_SHORT;
            break;
        default:
            err_usage(usestr);
            break;
        }
    }

    for (i = optind; i < argc; i++)
    {
        if (eval_realpath(argv[i]) != 0)
            rc = EXIT_FAILURE;
    }

    return(rc);
}

I needed it to test some software that was evaluating the security of a path, and needed to be sure my code was evaluating the given path to the same resolved location as realpath() does. It would probably be sensible to extend it with a '-a' option to ensure names are mapped to absolute names (by prefixing the result of getcwd() to relative pathnames).

(The extra source code is available in my SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c, stderr.h and errhelp.c in the src/libsoq sub-directory.)

Indictable

No, it does necessarily give you an absolute path name. The documentation says "The realpath() function shall derive, from the pathname pointed to by file_name, an absolute pathname that names the same file, ..." – Taraxacum

That's odd; you're right, yet...I must have gotten the wrong idea from somewhere. The difficulty almost four years after writing the code, and most of two years after writing the answer, is knowing where I got the misinformation from. It will take some investigation on obscure systems. – Indictable

Hi there. It's 2022. Trying to understand your answer. I do gcc -g -Wall reallink1.c stderr.c stderr.h it won't compile. The error is undefined reference to err_help'` / @JonathanLeffler – Majunga

@Mark: Apologies — you will need errhelp.c too , which is available from my SOQ repository, as documented in the tail of the question. If there's another non-standard file you need, the source/header should be in the same directory of the repository. If something is missing altogether, please let me know. – Indictable