As you can see, when you read file in /proc, any error could happen. E.g. When you read the status from a process, you may get ESRCH

proc_file_read_test.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <linux/limits.h>

#define ERROR_PRINTF(fmt, args...) {\
    fprintf(stderr, fmt, args); \
    _exit(EXIT_FAILURE); \
}

static int open_proc(const pid_t pid) {
    int fd;
    char path[PATH_MAX + 1];
    snprintf(path, PATH_MAX, "/proc/%ld/stat", (long) pid);
    path[PATH_MAX] = '\0';
    if ((fd = open(path, O_RDONLY)) < 0)
        ERROR_PRINTF("Cannot open %s: %s\n", path, strerror(errno));
    return fd;
}

static void read_proc(const int fd) {
    ssize_t n;
    char buf[256];
    while((n = read(fd, buf, 256))) {
        if (n < 0) {
            if (errno == EINTR)
                continue;
            else
                ERROR_PRINTF("Cannot read file %d: %s\n", fd, strerror(errno));
        }
        if (write(STDOUT_FILENO, buf, n) < 0)
            ERROR_PRINTF("Cannot output buf: %s\n", strerror(errno));
    }
}

static void ready_for_reread(const int fd) {
    if (lseek(fd, 0, SEEK_SET) < 0)
        ERROR_PRINTF("Cannot seek file: %s\n", strerror(errno));
}

static pid_t execute_command(char *argv[]) {
    pid_t pid;
    int dev_null;
    switch ((pid = fork())) {
    case -1:
        ERROR_PRINTF("Cannot fork: %s\n", strerror(errno));
    case 0:
        if (close(STDIN_FILENO) < 0)
            ERROR_PRINTF("Cannot close STDIN: %s\n", strerror(errno));
        if ((dev_null = open("/dev/null", O_WRONLY)) < 0)
            ERROR_PRINTF("Cannot open /dev/null: %s\n", strerror(errno));
        if (dup2(dev_null, STDOUT_FILENO) < 0 || dup2(dev_null, STDERR_FILENO) < 0)
            ERROR_PRINTF("Cannot dup /dev/null to STDOUT/STDERR: %s\n", strerror(errno));
        execvp(*argv, argv);
        ERROR_PRINTF("Cannot exec %s: %s\n", *argv, strerror(errno));
    default:
        return pid;
    }
}

static void wait_command(const pid_t pid) {
    // Never mind the command status
    if (waitpid(pid, NULL, 0) < 0)
        ERROR_PRINTF("Cannot wait command %ld: %s\n", (long) pid, strerror(errno));
}

int main(int argc, char const *argv[]) {
    char *command[] = {"find", "/", NULL};
    pid_t pid;
    int proc_fd;

    pid = execute_command(command);
    proc_fd = open_proc(pid);
    printf("1:\n");
    read_proc(proc_fd);
    ready_for_reread(proc_fd);
    sleep(1);
    printf("2:\n");
    read_proc(proc_fd);
    ready_for_reread(proc_fd);
    wait_command(pid);
    printf("3:\n");
    read_proc(proc_fd);

    return 0;
}