vlibc/src/stdio/popen.c

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

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

#include "stdio_impl.h"

#include "../internal/syscall.h"

/*
 * popen/pclose (POSIX base, todo 20): run "sh -c cmd" with a pipe
 * attached to its stdin ("w") or stdout ("r").
 *
 * The stream is a todo-15 FILE built on the FILE core. popen does NOT go
 * through the public fdopen(): todo 15's fdopen validates the requested
 * mode against the descriptor's F_GETFL access mode by masking with
 * O_RDWR, so a plain O_WRONLY descriptor is misread as O_RDONLY and a
 * "w" fdopen is rejected — and the pipe write end is exactly that case
 * (F_SETFL cannot promote a pipe end to O_RDWR). popen knows the access
 * mode of its own freshly created pipe end, so it goes straight to the
 * FILE core: stdio_parse_mode + stdio_alloc_file. Only the POSIX modes
 * "r" and "w" are accepted; the glibc "re"/"we" close-on-exec extension
 * is not. There is no sys/wait.h yet (todo 23): pclose waits via the
 * raw SYS_wait4, and the child exits via the raw SYS_exit_group when
 * the exec fails.
 *
 * Registry: a static table mapping FILE* to the shell pid. The todo-15
 * FILE struct has no spare field for the pid, so pclose looks the
 * stream up here. FOPEN_MAX (16) slots; a full table fails with EMFILE.
 * Not thread-safe — fine while there is no thread runtime (todo 45).
 *
 * pclose closes the stream first, then waits: for "w" streams the close
 * flushes the buffered writes and delivers EOF, letting the shell exit;
 * waiting first could deadlock on a full pipe. A stream not opened by
 * popen is still closed, and pclose returns -1 as POSIX requires. A wait
 * failure (ECHILD) returns -1 with errno set.
 */

struct vlibc_popen_entry
{
    FILE *stream;
    pid_t pid;
};

static struct vlibc_popen_entry popen_table[FOPEN_MAX];

// NOLINTBEGIN(bugprone-easily-swappable-parameters)
FILE *
popen(const char *command, const char *mode)
{
    int fds[2] = {0}; /* written by SYS_pipe; the analyzer cannot model it */
    int parent_fd;
    int child_fd;
    int read_mode;
    int m;
    int oflags;
    pid_t pid;
    FILE *f;
    int i;

    if (mode[0] == 'r' && mode[1] == '\0')
    {
        read_mode = 1;
    }
    else if (mode[0] == 'w' && mode[1] == '\0')
    {
        read_mode = 0;
    }
    else
    {
        errno = EINVAL;
        return 0;
    }
    (void)stdio_parse_mode(mode, &m, &oflags); /* "r"/"w" always parse */

    if (syscall_ret(__syscall1(SYS_pipe, (long)fds)) < 0)
    {
        return 0;
    }
    if (read_mode)
    {
        parent_fd = fds[0]; /* parent reads the shell's stdout */
        child_fd = fds[1];
    }
    else
    {
        parent_fd = fds[1]; /* parent writes the shell's stdin */
        child_fd = fds[0];
    }

    pid = fork();
    if (pid == 0)
    {
        char *sh_argv[4];
        int target = read_mode ? 1 : 0;

        /*
         * Child: wire the pipe end onto the shell's fd (0 for "w", 1
         * for "r"), close everything else. The guards cover the
         * (theoretical) case where the pipe handed us fd 0 or 1.
         */
        if (parent_fd != target)
        {
            (void)__syscall1(SYS_close, parent_fd);
        }
        if (child_fd != target)
        {
            (void)__syscall2(SYS_dup2, child_fd, target);
            (void)__syscall1(SYS_close, child_fd);
        }
        sh_argv[0] = "sh";
        sh_argv[1] = "-c";
        sh_argv[2] = (char *)command;
        sh_argv[3] = 0;
        execve("/bin/sh", sh_argv, environ);
        __syscall1(SYS_exit_group, 127); /* exec failed */
    }

    /* Parent. */
    (void)__syscall1(SYS_close, child_fd);
    if (pid < 0)
    {
        (void)__syscall1(SYS_close, parent_fd);
        return 0;
    }

    f = stdio_alloc_file(parent_fd, m);
    if (f == 0)
    {
        /* Drop the descriptor and reap the shell we just spawned. */
        (void)__syscall1(SYS_close, parent_fd);
        (void)__syscall4(SYS_wait4, pid, 0, 0, 0);
        return 0;
    }

    for (i = 0; i < FOPEN_MAX; i++)
    {
        if (popen_table[i].stream == 0)
        {
            popen_table[i].stream = f;
            popen_table[i].pid = pid;
            return f;
        }
    }

    /* Registry full: drop the stream and reap the shell. */
    (void)fclose(f);
    (void)__syscall4(SYS_wait4, pid, 0, 0, 0);
    errno = EMFILE;
    return 0;
}
// NOLINTEND(bugprone-easily-swappable-parameters)

int
pclose(FILE *stream)
{
    pid_t pid = 0;
    int found = 0;
    int status = 0;
    int i;

    for (i = 0; i < FOPEN_MAX; i++)
    {
        if (popen_table[i].stream == stream)
        {
            pid = popen_table[i].pid;
            popen_table[i].stream = 0;
            popen_table[i].pid = 0;
            found = 1;
            break;
        }
    }

    (void)fclose(stream);

    if (!found)
    {
        /* Not a popen stream: POSIX wants -1. The stream is still
         * closed. */
        return -1;
    }

    for (;;)
    {
        long r = __syscall4(SYS_wait4, pid, (long)&status, 0, 0);

        if (r < 0 && -r == EINTR)
        {
            continue;
        }
        if (r < 0)
        {
            return syscall_ret(r); /* -1 + errno (e.g. ECHILD) */
        }
        return status;
    }
}