CODEWITHSUNDEEP
cposixrealpath
Ralph
Ralph

Reputation: 6399

Example of realpath function in C

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.

Upvotes: 21

Views: 71592

Answers (3)

Jonathan Leffler
Jonathan Leffler

Reputation: 755094

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.)

Upvotes: 4

pmg
pmg

Reputation: 108986

  • Note

    • The realpath() function is not described in the C Standard

  • 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;
}

Upvotes: 32

oHo
oHo

Reputation: 54701

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

  • for compilation and link
  • 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

GitHub mirror repository

Upvotes: 5

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