程序编译后执行的结果，跟用gdb单步调试的结果不一样，求解？？

Linux_WSNs

用C代码做测试脚本，gdb单步是pass，然而执行却是fail，可能是什么原因引起的？等待高手

koolcoy

恭喜lz，俺之前也遇到过这样的情况{:3_192:}

鸡丝拌面

多进/线程?

Linux_WSNs

回复 3# 鸡丝拌面


    用到fork函数   child=fork(); if(child==0){.........}
执行时能进入if，但gdb调试时不能进入

zylthinking

晕， 查查那个 fork_follow_mode(或者 follow_fork_mode)???

雨过白鹭洲

By default, when a program forks, gdb will continue to debug the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode mode
Set the debugger response to a program call of fork or vfork. A call to fork or vfork creates a new process. The mode argument can be:
parent
The original process is debugged after a fork. The child process runs unimpeded. This is the default.
child
The new process is debugged after a fork. The parent process runs unimpeded.

http://sourceware.org/gdb/onlinedocs/gdb/Forks.html

Linux_WSNs

#define _XOPEN_SOURCE 600

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include "posixtest.h"

#define TNAME "mmap/11-4.c"

int main()
{
  char tmpfname[256];
  long  page_size;
  long total_size;

  void *pa = NULL, *pa_2 = NULL;
  void *addr = NULL;
  size_t len;
  int flag;
  int fd, fd_2;
  off_t off = 0;
  int prot;

  pid_t child;
  char *ch = NULL, *ch_2 = NULL;
int exit_val;

  page_size = sysconf(_SC_PAGE_SIZE);

  /* Size of the file to be mapped */
  total_size = page_size / 2;

  /* mmap will create a partial page */
  len = page_size / 2;

  snprintf(tmpfname, sizeof(tmpfname), "/tmp/pts_mmap_11_5_%d",
             getpid());
  child = fork();
  if (child == 0)
  {
    /* Create shared object */
    unlink(tmpfname);
    fd = open(tmpfname, O_CREAT | O_RDWR | O_EXCL,
              S_IRUSR | S_IWUSR);
    if (fd == -1)
    {
      printf(TNAME " Error at open(): %s\n",
             strerror(errno));
      exit(PTS_UNRESOLVED);
    }
    if(ftruncate(fd, total_size) == -1) {
      printf(TNAME " Error at ftruncate(): %s\n", strerror(errno));
      return PTS_UNRESOLVED;
    }

    prot = PROT_READ | PROT_WRITE;
    flag = MAP_SHARED;
off = 0;
    pa = mmap(addr, len, prot, flag, fd, off);
    if (pa == MAP_FAILED)
    {
      printf("Test FAIL: " TNAME " Error at mmap(): %s\n",
             strerror(errno));
      return PTS_FAIL;
    }
    printf("pa: %p\n", pa);
    /* Check the patial page is ZERO filled */
    ch = pa + len + 1;
    if (*ch != 0)
    {
      printf("Test Fail: " TNAME " The partial page at the end of an object "
              "is not zero-filled\n");
      return PTS_FAIL;
    }

    /* Write the partial page */
    *ch = 'b';
    munmap (pa, len);
    close (fd);
    return PTS_PASS;

  }
  wait(&exit_val);
  if (!(WIFEXITED(exit_val) && (WEXITSTATUS(exit_val) == PTS_PASS)))
{
    unlink(tmpfname);
    return PTS_FAIL;
  }

  fd_2 = open(tmpfname, O_RDWR, 0);
  unlink(tmpfname);

  prot = PROT_READ | PROT_WRITE;
  flag = MAP_SHARED;
  off = 0;
  pa = mmap(addr, len, prot, flag, fd_2, off);
  pa_2 = mmap(addr, len, prot, flag, fd_2, off);
  if (pa_2 == MAP_FAILED)
  {
    printf("Test FAIL: " TNAME " Error at 2nd mmap(): %s\n",
            strerror(errno));
    exit(PTS_FAIL);
  }

  printf("pa_2: %p\n", pa_2);
  ch_2 = pa_2 + len + 1;
  if (*ch_2 == 'b')
  {
        printf("Test Fail: " TNAME " Modification of the partial page "
           "at the end of an object is written out\n");
        exit(PTS_FAIL);
  }
  close (fd_2);
  munmap (pa_2, len);
  printf("Test Passed\n");
  return PTS_PASS;
}

输出结果为：pa: 0xb77c4000
pa_2: 0xb77c3000
Test Fail: mmap/11-4.c Modification of the partial page at the end of an object is written out