c - 使用 Make 编译 C 代码时出错 'ld: symbol(s) not found for architecture x86_64'

Question

我尝试在 OS X 上使用 Make 编译 C 程序，但收到错误消息。

C 代码

#define _XOPEN_SOURCE // required for cuserid to work

// includes
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/resource.h>
#include <time.h>

// user-defined constants
#define FIB_MAX_ITERS 20
#define SLEEP_LENGTH 2

// Prints an error message and exits if one occurs. Else, returns the system call value.
int print_if_err(int syscall_val)
{
    if (syscall_val < 0) {
        perror("fork");
        exit(errno);
    } else {
        // No syscall error; we can return.
        return syscall_val;
    }
}

char* cuserid_wrapper()
{
    char* val = cuserid(NULL);
    if (val == NULL) {
        perror("cuserid");
        exit(errno);
    } else {
        return val;
    }
}

time_t time_wrapper()
{
    time_t val = time(NULL);
    if (val == ((time_t) -1)) {
        perror("time_wrapper");
        exit(errno);
    } else {
        return val;
    }
}

void getrusage_wrapper(struct rusage* output)
{
    int val = getrusage(RUSAGE_SELF, output);
    if (val == -1){
        perror("getrusage_wrapper");
        exit(errno);
    }
}

void print_uids_gids(const char* proc_name)
{
    printf("[%s] username: %s\n", proc_name, cuserid_wrapper());

    // According to man pages, following syscalls always succeed. 
    // Hence, no error checking
    printf("[%s] user id: %d\n",  proc_name, getuid()); 
    printf("[%s] effective user id: %d\n", proc_name, geteuid());
    printf("[%s] group id: %d\n",  proc_name, getgid());
    printf("[%s] effective group id: %d\n", proc_name, getegid());
    printf("[%s] pid: %d\n", proc_name, getpid());
}

void print_exec_times(const char* proc_name)
{
    // build up some cpu time
    int k = 0;
    int i;
    for (i = 0; i < 100000000; i++) {
        k += 2;
    }

    time_t cur_time;
    struct rusage res_usage;

    cur_time = time_wrapper();
    getrusage_wrapper(&res_usage);

    printf("[%s] seconds since epoch: %d\n", proc_name, (int)cur_time);
    printf("[%s] current time: %s", proc_name, ctime(&cur_time));
    printf("[%s] user CPU time: %d us\n", proc_name, (int)res_usage.ru_utime.tv_usec);
    printf("[%s] system CPU time: %d us\n", proc_name, (int)res_usage.ru_stime.tv_usec);
}

void sleep_wrapper(unsigned int length)
{
    int time_left = length;
    while (time_left > 0) {
        time_left = sleep(length);
    }
}

char* getcwd_wrapper()
{
    char* cwd = getcwd(NULL, 0);
    if (cwd == NULL) {
        perror("getcwd_wrapper");
        exit(errno);
    } else {
        return cwd;
    }
}

void waitpid_wrapper(pid_t pid)
{
    int status;
    int val = waitpid(pid, &status, 0);
    if (val == -1) {
        perror("waitpid_wrapper");
        exit(errno);
    } 
}

void child_proc(int fib_start, const char* proc_name)
{
    print_uids_gids(proc_name);
    // No errors can occur for getppid
    printf("[%s] parent PID: %d\n", proc_name, getppid());

    sleep(SLEEP_LENGTH); // sleep to align fibonacci prints
    // main loop - printing fibonacci numbers
    int f_prev = 1; // starts at f_1
    int f_cur = 1; // starts at f_2
    int f_cur_new;
    int f_prev_new;
    int i;
    for (i = fib_start; i <= FIB_MAX_ITERS; i += 2) { // want <= b/c 1-based index
        if (i % 2 == 1) {
            printf("[%s] f_%d = %d\n", proc_name, i, f_prev);
        } else if (i % 2 == 0) {
            printf("[%s] f_%d = %d\n", proc_name, i, f_cur);
        }

        f_cur_new  =  2 * f_cur + f_prev;
        f_prev_new = f_cur + f_prev;
        f_cur  = f_cur_new;
        f_prev = f_prev_new;

        sleep(SLEEP_LENGTH);
    }

    print_exec_times(proc_name);
    exit(0);
}

char* get_proc_name(const char* base, char* buff)
{
    sprintf(buff, "%s, PID=%d", base, (int)getpid());
    return (char*)buff;
}

int main(void)
{
    char parent_proc_name[255];
    get_proc_name("parent", parent_proc_name);
    // TODO: Error check the syscalls!
    printf("[%s] current working directory: %s\n",  parent_proc_name, getcwd_wrapper());
    print_uids_gids(parent_proc_name);

    pid_t child1_pid = print_if_err(fork());
    if (child1_pid == 0) {
        sleep(SLEEP_LENGTH / 2);

        char proc_name[255];
        get_proc_name("child1", proc_name);
        child_proc(1, proc_name);
    }
    printf("[%s] child1_pid: %d\n", parent_proc_name, child1_pid);

    pid_t child2_pid = print_if_err(fork());
    if (child2_pid == 0) {
        sleep(SLEEP_LENGTH);

        char proc_name[255];
        get_proc_name("child2", proc_name);
        child_proc(2, proc_name);
    }
    printf("[%s] child2_pid: %d\n", parent_proc_name, child1_pid);

    int status;
    // TODO: Error check the syscalls!
    waitpid(child1_pid, &status, 0);
    printf("[%s] child1 terminated\n", parent_proc_name);
    waitpid(child2_pid, &status, 0);
    printf("[%s] child2 terminated\n", parent_proc_name);
    print_exec_times(parent_proc_name);
    return 0;
}

制作文件

CC = gcc
C_FLAGS = -Wall -Wextra

all: eecs338_hw01

eecs338_hw01: eecs338_hw01.o
    $(CC) eecs338_hw01.o -o eecs338_hw01

eecs338_hw01.o: eecs338_hw01.c
    $(CC) -c $(C_FLAGS) eecs338_hw01.c

clean:
    rm -f eecs338_hw01 eecs338_hw01.o

错误

gcc eecs338_hw01.o -o eecs338_hw01
Undefined symbols for architecture x86_64:
  "_cuserid", referenced from:
      _cuserid_wrapper in eecs338_hw01.o
     (maybe you meant: _cuserid_wrapper)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [eecs338_hw01] Error 1

Answer 1

您应该设置#define _XOPEN_SOURCE 700(或600)，而不是简单地定义它。但是，这不会影响链接，只会影响符号在标题中是否可见。

在 Mac OS X 上运行 man cuserid 显示该函数没有手册页。

使用我的常规选项编译您的代码:

 gcc -O3 -g -std=c11 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
     -Wold-style-definition -Werror cuserid.c -o cuserid

要求我对您的代码进行一些细微的更改。这些函数被设为静态，因此不需要单独的原型(prototype)，并且函数定义使用显式void来指示没有参数:

char* cuserid_wrapper(void)

而不是:

char* cuserid_wrapper()

这些都是相对较小的问题(当您不使用如此严格的编译选项进行开发时或多或少是正常的)。进行这些更改后，我被告知 sleep_wrapper() 和 waitpid_wrapper() 已定义但未使用(再次证明这不是 MCVE)。我还得到:

cuserid.c:33:17: error: implicit declaration of function 'cuserid' [-Werror=implicit-function-declaration].

鉴于此，链接失败也就不足为奇了；系统不知道 cuserid() 函数。我发现 cuserid() 当前不属于 POSIX 的一部分。 cuserid() 的 Linux 手册页注释:

System V has a cuserid() function which uses the real user ID rather than the effective user ID. The cuserid() function was included in the 1988 version of POSIX, but removed from the 1990 version. It was present in SUSv2, but removed in POSIX.1-2001.

TL;DR

cuserid() 函数不可移植，并且无法移植到 Mac OS X。