How do I check if a file is executable using ANSI C?

Question

I'm trying to write a program that checks files in a directory. When a new file is created I have to check if it executable and if so I have to execute it.

I'm using the inotify interface and it works well, but I have some problems when I try to check whether a file is executable using stat(2). I found that I don't have permission to execute it.

Does passing to the program an absolute path of the directory that I want to check create permissions problems?

int main(int argc,char * argv []){

int fd,wd,len,i=0;
char buffer [EVENT_SIZE_BUF];

if(argc != 2)
    usage();

if((fd = inotify_init()) == -1)
    perror("inotify_init()");   


if((wd = inotify_add_watch(fd,argv[1],IN_CREATE) -1))
    perror("inotify_add_watch()");

while(1){


    if((len = read(fd,buffer,EVENT_SIZE_BUF)) < 0)
        perror("read()");

    struct inotify_event * ev = (struct inotify_event *)&buffer;

    if(ev->len > 0){

        if(ev->mask & IN_CREATE && ((ev-> mask & IN_ISDIR) == 0x00)){

            printf("SPY; new file is created %s\n",ev->name);

            char * path = strcat(argv[1],ev->name);
            printf("%s\n",path);

            struct stat sb;

            if(!stat(path,&sb)){

                printf( (S_ISDIR(sb.st_mode)) ? "d" : "-");
                printf( (sb.st_mode & S_IRUSR) ? "r" : "-");
                printf( (sb.st_mode & S_IWUSR) ? "w" : "-");
                printf( (sb.st_mode & S_IXUSR) ? "x" : "-");
                printf( (sb.st_mode & S_IRGRP) ? "r" : "-");
                printf( (sb.st_mode & S_IWGRP) ? "w" : "-");
                printf( (sb.st_mode & S_IXGRP) ? "x" : "-");
                printf( (sb.st_mode & S_IROTH) ? "r" : "-");
                printf( (sb.st_mode & S_IWOTH) ? "w" : "-");
                printf( (sb.st_mode & S_IXOTH) ? "x" : "-");
                fflush(stdout);
                printf("\n");
            }




        }else{ printf("dir\n"); }
    }               
}

inotify_rm_watch(fd,wd);
close(fd);
return 0;
}

Answer 1

This is impossible in ANSI C, you need POSIX.

Answer 2

Are you solving Renzo Davoli's exercises? :)

However, I was stuck likes you and I've discovered that copying an executable with Nautilus, it creates a "Executable (Copy)" that returns X-Permission error with stat() or access().

Copying with cp cp exe1 exe2 it works!!

This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/inotify.h>
#include <sys/stat.h>

#define _GNU_SOURCE 
#define EVENT_SIZE  ( sizeof (struct inotify_event) )
#define EVENT_BUF_LEN     ( 1024 * ( EVENT_SIZE + 16 ) )

void main(int argc, char **argv){

int fd, wd, length, i=0;
char buffer[EVENT_BUF_LEN];

char* dir = argv[1];

/*creating the INOTIFY instance*/
if((fd = inotify_init()) < 0)
    perror( "inotify_init" );


/*adding the directory into watch list. 
Here, the suggestion is to validate the existence of the directory before adding into monitoring list.*/
wd = inotify_add_watch( fd, dir, IN_CREATE | IN_DELETE );

while(1) {

    i=0;
    pid_t p;
    struct stat st;

    /*read to determine the event change happens on “/tmp” directory. Actually this read blocks until the change event occurs*/ 
    if((length = read(fd, buffer, EVENT_BUF_LEN )) < 0)
        perror( "read" );
    /*actually read return the list of change events happens. Here, read the change event one by one and process it accordingly.*/
    while ( i < length ){     
        struct inotify_event *event = ( struct inotify_event * ) &buffer[ i ];     
        if ( event->len ) {
            if ( event->mask & IN_CREATE ) {
                if ( event->mask & IN_ISDIR )
                    printf("New directory %s created.\n", event->name );
                else{
                    printf("New file %s created.\n", event->name );
                    char* file_name;
                    asprintf(&file_name, "%s/%s", dir, event->name);

                    printf("Check if %s is executable\n", file_name);
                    if (access(file_name, X_OK) == 0){ //If file is executable
                        printf("Executable file! Call fork()\n");
                        if ((p = fork()) == 0) {  // Child process: do your work here
                            if (execlp(file_name, file_name,  (char*) 0)<0)
                                fprintf(stderr, "execlp error\n");
                            exit(0);
                        }
                        else{
                            while(waitpid(p, NULL, WNOHANG) != p);
                            remove (event->name);
                            printf("%s done & gone\n", event->name);
                        }
                    }
                    else
                        fprintf(stderr, "Not executable file\n");
                }
            }
            else if ( event->mask & IN_DELETE ) {
                if ( event->mask & IN_ISDIR ) 
                    printf( "Directory %s deleted.\n", event->name );
                else
                    printf( "File %s deleted.\n", event->name );
            }
        }

        i += EVENT_SIZE + event->len;
    }
}

/*removing the “/tmp” directory from the watch list.*/
inotify_rm_watch( fd, wd );

/*closing the INOTIFY instance*/
close( fd );

 }

Answer 3

You are using linux, so you can use access() function

if (access("/my/file", X_OK) != -1) {
    printf("execution permission enabled\n");
} else {
    printf("execution permission disabled\n");
}