How to handle errors in execvp? - c

Question

I've written a small program (with code from SO) that facilitates printenv | sort | less. Now I want to implement error-handling and I start with execvp. Is it just to check the return value and what more? AFAIK I just check the return value if it was 0 in this function return execvp (cmd [i].argv [0], (char * const *)cmd [i].argv);. Is that correct?
#include <sys/types.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
struct command
{
const char **argv;
};
/* Helper function that spawns processes */
int spawn_proc (int in, int out, struct command *cmd) {
pid_t pid;
if ((pid = fork ()) == 0) {
if (in != 0) {
dup2 (in, 0);
close (in);
}
if (out != 1) {
dup2 (out, 1);
close (out);
}
return execvp (cmd->argv [0], (char * const *)cmd->argv);
}
return pid;
}
/* Helper function that forks pipes */
int fork_pipes (int n, struct command *cmd) {
int i;
int in, fd [2];
for (i = 0; i < n - 1; ++i) {
pipe (fd);
spawn_proc (in, fd [1], cmd + i);
close (fd [1]);
in = fd [0];
}
dup2 (in, 0);
return execvp (cmd [i].argv [0], (char * const *)cmd [i].argv);
}
int main (int argc, char ** argv) {
int i;
if (argc == 1) { /* There were no arguments */
const char *printenv[] = { "printenv", 0};
const char *sort[] = { "sort", 0 };
const char *less[] = { "less", 0 };
struct command cmd [] = { {printenv}, {sort}, {less} };
return fork_pipes (3, cmd);
}
if (argc > 1) { /* I'd like an argument */
if (strncmp(argv[1], "cd", 2) && strncmp(argv[1], "exit", 2)) {
char *tmp;
int len = 1;
for( i=1; i<argc; i++)
{
len += strlen(argv[i]) + 2;
}
tmp = (char*) malloc(len);
tmp[0] = '\0';
int pos = 0;
for( i=1; i<argc; i++)
{
pos += sprintf(tmp+pos, "%s%s", (i==1?"":"|"), argv[i]);
}
const char *printenv[] = { "printenv", 0};
const char *grep[] = { "grep", "-E", tmp, NULL};
const char *sort[] = { "sort", 0 };
const char *less[] = { "less", 0 };
struct command cmd [] = { {printenv}, {grep}, {sort}, {less} };
return fork_pipes (4, cmd);
free(tmp);
} else if (! strncmp(argv[1], "cd", 2)) { /* change directory */
printf("change directory to %s\n" , argv[2]);
chdir(argv[2]);
} else if (! strncmp(argv[1], "exit", 2)) { /* change directory */
printf("exit\n");
exit(0);
}
}
exit(0);
}

Answer 1

Look at the manual : execvp change your process's program image to another. So if there is a return value at all, it will be obviously -1, because it will have obviously failed.
As for any syscall or functions calling syscall, execvp with set errno for an appropriate error code, that you can use with perror to know why your function failed. (By the way, you should do the same to any syscall: fork, pipe and dup2 at least.)
Then, if by "error handling", you mean handle errors from the program that you launched, you can make it with wait or waitpid. It will allow you to know how the program ended, it return value, if it ended by a signal, if yes wich one, etc.