Attempting to pass Linux command output into a variable

I have the following code:

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

int readCmd(char* cmd) {
    FILE *fp;
    char path[1024];

    /* Open the command for reading. */
    fp = popen(cmd, "r");
    if (fp == NULL) {
        printf("Failed to run commandn" );
        exit(1);
    }

    /* Read the output a line at a time - output it. */
    while (fgets(path, sizeof(path)-1, fp) != NULL) {
        printf("%s", path);
    }
    /* close */
    pclose(fp);
    return path;
}


int main( int argc, char *argv[] ) {
char* gp123;
char* gp124;


gp124=readCmd("cat /sys/class/gpio/gpio124/direction");
gp123=readCmd("cat /sys/class/gpio/gpio123/direction");
printf("123: %s n124: %s n",gp123,gp124);

}

With the following output:

out
in
123: in
124: in

As you can see, gpio pin 123 is set to ‘in’ and 124 is set to ‘out’.

However, both gp123 and gp124 are being assigned ‘in’. I am rusty with functions and C. Could you fellas help me out a little please?

More specifically, why is it failing to assign the returned value for each command I read? Is this a local/global variable issue?

Thanks!

EDIT:

#include <stdio.h>
#include <stdlib.h>
char* readCmd(char* cmd)
{
FILE *fp;
static char path[1024];

/* Open the command for reading. */
fp = popen(cmd, "r");
if (fp == NULL) {
    printf("Failed to run commandn" );
    exit(1);
}

/* Read the output a line at a time - output it. */
while (fgets(path, sizeof(path)-1, fp) != NULL) {
    printf("%s", path);
}
/* close */
pclose(fp);
return path;
}


int main( int argc, char *argv[] )
{
    char gp123[50];
    char gp124[50];
    char *answer;

    answer=readCmd("cat /sys/class/gpio/gpio124/direction");
    sprintf(gp124, "%s", answer);
    answer=readCmd("cat /sys/class/gpio/gpio123/direction");
    sprintf(gp123, "%s", answer);
    printf("123: %s n124: %s n",gp123,gp124);

}

This seems to work correctly, is there anything here I should change to be more correct?

Answer

You can’t return a pointer to automatic memory from a function. The path array only exists for the duration of the readCmd function (which should return char*, not an int).

To solve it, you can:

• declare the array static (or use a global array–pretty much the same thing) and embrace non-reenterability
• use the heap (malloc the array or use the GNU getline function)
• ask the caller to provide provide a pointer to where to save the output (its size should be passed in too)