Getting FILE information Via Stat in C Language

We know that Linux/Unix file system stores information for every file. Some of the information can be obtain by stat - A Unix system call. This system call is defined in < sys/stat.h> header file.

stat structure has the following fields.

struct stat {
    dev_t     st_dev;     /* ID of device containing file */
    ino_t     st_ino;     /* inode number */
    mode_t    st_mode;    /* protection */
    nlink_t   st_nlink;   /* number of hard links */
    uid_t     st_uid;     /* user ID of owner */
    gid_t     st_gid;     /* group ID of owner */
    dev_t     st_rdev;    /* device ID (if special file) */
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for file system I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};


Now lets write a small program to get the file information using stat system call.

// File name Stat.c

#include <unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
int main(int argc, char **argv)
{
    if(argc != 2)   
        return 1;

    struct stat fileInfo;
    // getting file information into fileInfo using stat
    if(stat(argv[1],&fileInfo) < 0)   
        return 1;

    // printing file information
    printf("Information for %s\n",argv[1]);
    printf("---------------------------------------------------\n");
    printf("File Size: \t\t%d bytes\n",fileInfo.st_size);
    printf("Number of Links: \t%d\n",fileInfo.st_nlink);
    printf("File inode: \t\t%d\n",fileInfo.st_ino);

    // S_ISDIR, S_IRUSR,....are marcos
    printf("File Permissions Are: \t");
    printf( (S_ISDIR(fileInfo.st_mode)) ? "d" : "-");
    printf( (fileInfo.st_mode & S_IRUSR) ? "r" : "-");
    printf( (fileInfo.st_mode & S_IWUSR) ? "w" : "-");
    printf( (fileInfo.st_mode & S_IXUSR) ? "x" : "-");
    printf( (fileInfo.st_mode & S_IRGRP) ? "r" : "-");
    printf( (fileInfo.st_mode & S_IWGRP) ? "w" : "-");
    printf( (fileInfo.st_mode & S_IXGRP) ? "x" : "-");
    printf( (fileInfo.st_mode & S_IROTH) ? "r" : "-");
    printf( (fileInfo.st_mode & S_IWOTH) ? "w" : "-");
    printf( (fileInfo.st_mode & S_IXOTH) ? "x" : "-");
    printf("\n");

    return 0;
}


To Run: gcc Stat.c
              ./a.out Stat.c  // giving Stat.c file name as input

Out Put: 
Information for Stat.c
----------------------------------------
File Size:         1256 bytes
Number of Links:     1
File inode:         2367639
File Permissions Are:     -rw-r--r--

Redirection Using dup & dup2

dup and dup2 are basic I/O Unix system calls. Following program shows a sequence of dup operations that temporarily redirect the standard out put to "stdout.log" file. If we then want to use the original standard output we can duplicate that using dup2 system call, with help of already saved file descriptor fd.


// File Name: redirect.c

 #include <stdio.h>

   void main()
    {
        // File descriptor        
         int    fd;

        // This object is used to specify a position within a file     
         fpos_t pos;

        printf("\n stdout is normal ");
        fflush(stdout);

        //Getting current position in stream
        fgetpos(stdout, &pos);

        // Redirecting standard output to stdout.log file
        fd = dup(fileno(stdout));
        freopen("stdout.log", "w", stdout);

        fun();
        fflush(stdout);

        // Standard output is restoring to original settings
        dup2(fd, fileno(stdout));
        close(fd);
        clearerr(stdout);
        fsetpos(stdout, &pos);

        printf("\n stdout is normal again\n");
    }

    fun()
    {
    printf("stdout in f() is redirected to this file(stdout.log)");
    }

To run : gcc redirect.c
              ./a.out

After running above 2 commands you can see a file named stdout.log in your current directory with content "stdout in f() is redirected to this file(stdout.log)".

Run Applications Through C Program

Some times our project demands to start a particular application through C program. Here i will show you how to do that.

To start a application through C program, we can use a system call called execlp, which comes under exec family of UNIX system calls.

// Compiler: GCC
//File Name: execute.c
#include <stdio .h>
#include <unistd .h>
void main()
{
pid_t pid = fork();
switch (pid) {
       case 0:
        execlp("/bin/sh", "sh", "-c", "firefox http://umencs.blogspot.in", NULL);
            //execlp("/bin/sh", "sh", "-c", "gedit", NULL);
            //execlp("/bin/sh", "sh", "-c", "nautilus", NULL);
       case -1:
        printf("Error: can't execute command: fork() failed\n");
       }
}

In execlp system call -c option is used to read commands from the command_string operand instead of from the standard input.

To run above program use following commands
$gcc execute.c
$./a.out

the above program will open  http://umencs.blogspot.in in Firefox.