Run a command through popen and get the output.

run_cmd.c

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

size_t get_line_size() {
    // https://man7.org/linux/man-pages/man3/fgets.3p.html
    const size_t kLineMax = 16384u;
    size_t line_max;
    if (LINE_MAX >= kLineMax) {
        line_max = kLineMax;
    } else {
        long limit = sysconf(_SC_LINE_MAX);
        line_max =
            (limit < 0 || (size_t)limit > kLineMax) ? kLineMax : (size_t)limit;
    }
    return line_max + 1;
}

int run_cmd(const char* cmd, char* output, size_t n) {
    if (cmd == NULL) {
        return 0;
    }
    FILE* fp = popen(cmd, "r");
    if (fp == NULL) {
        fprintf(stderr, "failed. cmd=%s\n", cmd);
        perror("popen failed\n");
        return -1;
    }
    if (output == NULL || n == 0) {
        if (pclose(fp) == -1) {
            perror("pclose failed\n");
        }
        return -1;
    }
    size_t line_size = get_line_size();
    char* line = (char*)malloc(line_size);
    if (line == NULL) {
        fprintf(stderr, "malloc size=%ld failed\n", line_size);
        perror("malloc failed\n");
        if (pclose(fp) == -1) {
            perror("pclose failed\n");
        }
        return -1;
    }
    size_t size_remain = n;
    char* dest = output;
    while (size_remain > 0 && fgets(line, line_size, fp) != NULL) {
        size_t s = strlen(line);
        size_t size_copy = size_remain < s ? size_remain : s;
        memcpy(dest, line, size_copy);
        size_remain -= size_copy;
        dest += size_copy;
    }
    *dest = '\0';
    free(line);
    if (pclose(fp) == -1) {
        perror("pclose failed\n");
        return -1;
    }
    return 0;
}

int main() {
    // only execute the command
    run_cmd("ls", NULL, 0);
    run_cmd("ls", NULL, 1);
    run_cmd("ls", (char*)1, 0);

    // execute the command and print the result
    char output[LINE_MAX];
    run_cmd("ls /", output, LINE_MAX);
    printf("%s", output);

    return 0;
}