Popen 不起作用后 C 程序在 Fgets 上挂起

Question

编辑:我写了一个 C 程序，它生成可逆函数并将它们写入 R 文件中，这些文件用 popen 执行和打开。我在 Ubuntu 16.04 上运行这个程序。我的程序可以编译，但 R 脚本永远不会完成/挂起

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#define num_func 10

char * makeInvert(){
    char * buffer = malloc(200 * sizeof(char));
    double a = (double)(rand()%5)-5.0; 
    double b = (double)(rand()%5)-5.0; 
    double c = (double)(rand()%5)-5.0; 
    double n = (double)(rand()%5)-5.0;
    n *= 2;
    n += 1;
    sprintf(buffer, "%g(x + %g)^%g + %g == y", a, b, n, c);
    return buffer;

}

int main() {
  printf("entered main \n");
  int i;
  char * buff1;

  for (i = 0; i < num_func; i++) {
        char R[5000];
    buff1 = makeInvert();

    strcpy(R, "library(Ryacas)\n");
    strcat(R, "yacas(\"Solve(");
    strcat(R, buff1);
    strcat(R, ", x)\")\n\n");
    //sleep(1);
    char filename[100];
    strcpy(filename, "computation/");
    strcat(filename, "inverse");
    strcat(filename, ".R");
    FILE * f = fopen(filename, "w");    
    if (!f) {       //validate file is open 
            printf("cant open file");
     }
    fputs(R, f);
    printf("before fclose");
    fflush(stdout);
    fclose(f);
        f = NULL;
    printf("after fclose");
    fflush(stdout);

    char path[5000];
    char command[300];
    strcpy(command,"Rscript ");
    strcat(command, "computation/inverse");

    strcat(command, ".R");
    printf("command %s\n", command);
    FILE * fp = popen(command, "r");

    if (!fp) {       //validate file is open 
            printf("cant open file");
            fflush(stdout);
     }

    printf("after popen\n");
    fflush(stdout);

    while (fgets(path, sizeof(path)-1, fp) != NULL) {
           printf("output: %s\n", path);

        }
    fclose(fp);

  }
  return 0;
}

运行 strace -f program_name 的输出

execve("/usr/bin/test", ["test"], [/* 20 vars */]) = 0
brk(NULL)                               = 0x16ef000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f95f15a9000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=53425, ...}) = 0
mmap(NULL, 53425, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f95f159b000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\t\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f95f0fbc000
mprotect(0x7f95f117c000, 2097152, PROT_NONE) = 0
mmap(0x7f95f137c000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f95f137c000
mmap(0x7f95f1382000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f95f1382000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f95f159a000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f95f1599000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f95f1598000
arch_prctl(ARCH_SET_FS, 0x7f95f1599700) = 0
mprotect(0x7f95f137c000, 16384, PROT_READ) = 0
mprotect(0x60a000, 4096, PROT_READ)     = 0
mprotect(0x7f95f15ab000, 4096, PROT_READ) = 0
munmap(0x7f95f159b000, 53425)           = 0
brk(NULL)                               = 0x16ef000
brk(0x1710000)                          = 0x1710000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=1668976, ...}) = 0
mmap(NULL, 1668976, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f95f1400000
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(1)                           = ?
+++ exited with 1 +++

当我直接运行 Rscript computation/inverse.R 时，它挂起并且不输出任何内容。

---

我相信它卡在 fgets 上，因为当我在 gdb 上运行程序进行回溯时，我看到了这个:

after popen 
^C
Program received signal SIGINT, Interrupt.
0x00007ffff7b04230 in __read_nocancel ()
    at ../sysdeps/unix/syscall-template.S:84
84  ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) backtrace
#0  0x00007ffff7b04230 in __read_nocancel ()
    at ../sysdeps/unix/syscall-template.S:84
#1  0x00007ffff7a875e8 in _IO_new_file_underflow (fp=0x6034f0) at fileops.c:592
#2  0x00007ffff7a8860e in __GI__IO_default_uflow (fp=0x6034f0) at genops.c:413
#3  0x00007ffff7a7bc6a in __GI__IO_getline_info (fp=fp@entry=0x6034f0, 
    buf=buf@entry=0x7fffffffd1d0 "", n=4998, delim=delim@entry=10, 
    extract_delim=extract_delim@entry=1, eof=eof@entry=0x0) at iogetline.c:60
#4  0x00007ffff7a7bd78 in __GI__IO_getline (fp=fp@entry=0x6034f0, 
    buf=buf@entry=0x7fffffffd1d0 "", n=<optimized out>, delim=delim@entry=10, 
    extract_delim=extract_delim@entry=1) at iogetline.c:34
#5  0x00007ffff7a7ab7d in _IO_fgets (buf=0x7fffffffd1d0 "", n=<optimized out>, 
    fp=0x6034f0) at iofgets.c:53
#6  0x0000000000401288 in main () at test.c:123
(gdb) frame 6
#6  0x0000000000401288 in main () at test.c:123
123     while (fgets(path, sizeof(path)-1, fp) != NULL) {

看完this post ，我在 popen

之后添加了这段代码

    int fd = fileno(fp);

    int flags;
    flags = fcntl(fd, F_GETFL, 0);
    flags |= O_NONBLOCK;
    fcntl(fd, F_SETFL, flags);

该帖子解释说，“在 Linux(或任何 Unix-y 操作系统)中，您可以将 popen() 使用的底层文件描述符标记为非阻塞。[使用上面的代码] 如果没有可用的输入, fgets 将返回 NULL 并将 errno 设置为 EWOULDBLOCK。”当我现在运行 gdb 时，我得到:

after popen 
^C
Program received signal SIGINT, Interrupt.
0x00007ffff785f188 in _IO_new_proc_close (fp=0x6034f0) at iopopen.c:339
339 iopopen.c: No such file or directory.
(gdb) bt
#0  0x00007ffff785f188 in _IO_new_proc_close (fp=0x6034f0) at iopopen.c:339
#1  0x00007ffff7869960 in _IO_new_file_close_it (fp=fp@entry=0x6034f0) at fileops.c:172
#2  0x00007ffff785d3ef in _IO_new_fclose (fp=0x6034f0) at iofclose.c:58
#3  0x00000000004013f9 in main () at test.c:130
(gdb) frame 3
#3  0x00000000004013f9 in main () at test.c:130
130     fclose(fp);
(gdb) 

Answer 1

我没有检查整个代码，但我发现了这个缺陷:

char R[5000];
buff1 = makeInvert();
strcat(R, "library(Ryacas)\n");
strcat(R, "yacas(\"Solve(");

您 strcat 将“library...”添加到未初始化的字符串中。您应该像在其他地方那样对第一个字符串使用 strcpy 或插入 R[0]='\0';。