#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>
#include <limits.h>

#define MIN(x,y)    (((x) < (y)) ? (x) : (y))

char * rpath_to_apath(const char * rpath, char * buf, int buf_size);

int main(int argc, char * argv[])
{
    char * rpath = NULL;
    char
 
 * apath = NULL;

    if((argc == 2 && strcmp(argv[1], "--help") == 0)
        || argc > 2)
    {
        printf("Usage: %s <relative_path>\n", argv[0]);
        return EXIT_SUCCESS;
    }

    if(argc == 1)
        rpath = "./";
    else
        rpath = argv[1];

    if(rpath[0] == '/')
    {
        fprintf
 
(stderr, "path must be relative path!\n");
        return EXIT_FAILURE;
    }
    printf("relative path: %s\n", rpath);

    if((apath = malloc(PATH_MAX)) == NULL)
    {
        perror("malloc");
        return EXIT_FAILURE;
    }
    memset(apath, 0, PATH_MAX);

    printf("absolute path: %s\n", rpath_to_apath(rpath, apath, PATH_MAX));
    free
 
(apath);

    return EXIT_SUCCESS;
}

char * rpath_to_apath(const char * rpath, char * buf, int buf_size)
{
    char cur_work_dir[PATH_MAX] = {0};
    int index = 0;
    const char * p = rpath;

    while(*p == '.')
        p++;
    if(*p == '/')
        p++;

    if(getcwd(cur_work_dir, PATH_MAX) == NULL)
    {
        perror("getcwd");
        return NULL;
    }

    index = strlen(strncpy(buf, cur_work_dir, buf_size - 1));
    if(*p)
    {
        buf[index] = '/';
        strncpy(buf + index + 1, p,
                MIN(strlen(p), (buf_size - 1) - (index + 1)));
    }

    return buf;
}

這段程式碼僅僅是實現相對路徑和絕對路徑之間的轉換，但其中存在一個BUG：程式碼嚴重依賴於當前工作目錄。
正確的做法是呼叫系統呼叫realpath():

#include <stdlib.h>
char * realpath(const char * pathname, char * resolved_path);