1

通过 read(如 cat)或 fread(如 nl)读取 /proc/stat 文件的程序返回不同的值。read() /proc/stat 返回正确的结果,而 fread() 返回错误的结果。

我编写了简单的 C 程序,它们只读取或读取 /proc/stat。read() 返回正确的值,例如cat /proc/stat. fread() 返回错误的值,例如nl /proc/stat.

我在我的Centos7系统中执行“top”命令,然后按“1”显示每个cpu的详细信息,它只显示cpu0的信息,而我的系统有8个cpu核心。此外,cpu0的信息是错误的。

于是我阅读了top命令的源代码,发现它使用fread()从/proc/stat获取cpu信息。我将文件读取接口从glibc fread()函数更改为read(),它运行良好。

我使用 fread() 来检索 /proc 中的其他文件,例如 /proc/meminfo,它也很好用。

  1. fread() 从 /proc/stat 获取信息(命令 'nl' 使用 fread() 读取文件中的内容)</li>
[root@pro_app_server workspace]# nl /proc/stat
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     8  procs_blocked 0
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   110  processes 2774
   111  procs_running 4
   112  procs_blocked 0
   113  softirq 188202 0 129718 1 457 0 0 1 35688 0 22337cpu  0 0 3375 498153 97 0 10 0 0 0
   114  cpu0 0 0 1699 249246 71 0 6 0 0 0
   115  intr 0 28 0 0 0 378 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 16 0 4129 0 124 1 0 1251 0 3 0 6879 6680 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
   116  ctxt 543672
   117  btime 1544675938
   118  processes 2774
   119  procs_running 4
   120  procs_blocked 0
   121  softirq 188202 0 129718 1 457 0 0 1 35688 0 22337cpu  0 0 3375 498153 97 0 10 0 0 0
   122  cpu0 0 0 1699 249246 71 0 6 0 0 0
   123  intr 0 28 0 0 0 378 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 16 0 4129 0 124 1 0 1251 0 3 0 6879 6680 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
   124  ctxt 543672
   125  btime 1544675938
   126  processes 2774
   127  procs_running 4
   128  procs_blocked 0
   129  softirq 188202 0 129718 1 457 0 0 1 35688 0 22337
[root@pro_app_server workspace]#
  1. read() 从 /proc/stat 获取信息(命令 'cat' 使用 read() 读取文件中的内容)</li>
[root@pro_app_server workspace]# cat -n /proc/stat
     1  cpu  2058017 66 399693 188962556 16705 0 18516 5046 0 0
     2  cpu0 192984 3 47418 23679955 1327 0 3599 658 0 0
     3  cpu1 360654 16 59590 23478307 3861 0 2961 850 0 0
     4  cpu2 322760 24 53901 23533358 3234 0 2684 736 0 0
     5  cpu3 271322 4 52696 23600674 2217 0 2168 660 0 0
     6  cpu4 246241 13 48638 23643590 1678 0 1918 557 0 0
     7  cpu5 236649 2 46872 23660881 1362 0 1772 553 0 0
     8  cpu6 208005 2 45099 23691054 1219 0 1718 484 0 0
     9  cpu7 219399 1 45476 23674734 1804 0 1694 545 0 0
    10  intr 731124153 637 15753 0 0 0 0 3 0 0 0 0 34 44262 0 0 0 0 0 0 0 0 0 0 0 0 74547 0 0 358100 16030715 243 0 24 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
    11  ctxt 698139400
    12  btime 1559549547
    13  processes 1436019
    14  procs_running 2
    15  procs_blocked 0
    16  softirq 240304636 1 96899354 179 55128202 0 0 6370 22706849 0 65563681
[root@pro_app_server workspace]#
  1. 用户 read() 和 fread() 检索/proc下的其他文件,看起来很正常

  2. 我的带有 read() 的 C 程序

[root@pro_app_server workspace]# cat readstat.c -n
     1  #include <stdio.h>
     2  #include <stdlib.h>
     3  #include <sys/types.h>
     4  #include <sys/stat.h>
     5  #include <fcntl.h>
     6
     7  #define BUF_INC_SIZE 1024
     8
     9  int main()
    10  {
    11          int fd = 0;
    12          FILE *file = NULL;
    13          char *buf =NULL;
    14          size_t buf_size = 0, tot_read = 0, sgl_read = 0;
    15
    16          fd = open("/proc/stat", O_RDONLY);
    17          if(fd < 0)
    18                  perror("read /proc/stat fail!");
    19
    20          buf = malloc(BUF_INC_SIZE);
    21          if(!buf)
    22                  perror("malloc fail!");
    23          buf_size += BUF_INC_SIZE;
    24
    25          while(-1 != (sgl_read = read(fd, buf + tot_read, buf_size - tot_read)))
    26          {
    27                  tot_read += sgl_read;
    28                  if(tot_read < buf_size)
    29                          break;
    30
    31                  buf = realloc(buf, (buf_size += BUF_INC_SIZE));
    32          }
    33
    34          free(buf);
    35          close(fd);
    36          return 0;
    37  }
[root@pro_app_server workspace]#

  1. 我的带有 fread() 的 C 程序

[root@pro_app_server workspace]# cat freadstat.c -n
     1  #include <stdio.h>
     2  #include <stdlib.h>
     3
     4  #define BUF_INC_SIZE 1024
     5
     6  int main()
     7  {
     8          FILE *file = NULL;
     9          char *buf =NULL;
    10          size_t buf_size = 0, tot_read = 0, sgl_read = 0;
    11
    12          file = fopen("/proc/stat", "r+");
    13          if(!file)
    14                  perror("read /proc/stat fail!");
    15
    16          buf = malloc(BUF_INC_SIZE);
    17          if(!buf)
    18                  perror("malloc fail!");
    19
    20          buf_size += BUF_INC_SIZE;
    21
    22          while(0 < (sgl_read = fread(buf + tot_read, 1, buf_size - tot_read, file)))
    23          {
    24                  tot_read += sgl_read;
    25                  if(tot_read < buf_size)
    26                          break;
    27
    28                  buf = realloc(buf, (buf_size += BUF_INC_SIZE));
    29          }
    30
    31          free(buf);
    32          fclose(file);
    33          return 0;
    34  }
[root@pro_app_server workspace]#

作为 aschepler 的顾问,我跟踪我的 C 程序。:

6、read() /proc/stat 的 strace 信息(部分结果):


[root@pro_app_server workspace]# strace ./readstat
execve("./readstat", ["./readstat"], [/* 23 vars */]) = 0
brk(0)                                  = 0xcfb000
.
.
.
.
open("/proc/stat", O_RDONLY)            = 3
read(3, "cpu  2275345 66 527271 203616681"..., 1024) = 1024
read(3, "0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 "..., 1024) = 602
close(3)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++
[root@pro_app_server workspace]#


7、fread() /proc/stat 的 strace 信息(部分结果)


[root@pro_app_server workspace]# strace ./freadstat
execve("./freadstat", ["./freadstat"], [/* 23 vars */]) = 0
brk(0)                                  = 0xdeb000
.
.
.
open("/tmp/tmpfeQoAKy", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/tmpfeQoAKy")               = 0
fcntl(3, F_GETFL)                       = 0x8002 (flags O_RDWR|O_LARGEFILE)
open("/proc/stat", O_RDONLY)            = 4
fstat(4, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f7692850000
read(4, "cpu  2277177 66 528841 203745262"..., 1024) = 1024
fstat(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f769284f000
write(3, "cpu  0 0 3375 498153 97 0 10 0 0"..., 4096) = 4096
write(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
read(4, "0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 "..., 1024) = 602
write(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
write(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
read(4, "", 1024)                       = 0
close(4)                                = 0
munmap(0x7f7692850000, 4096)            = 0
write(3, "5938\nprocesses 2774\nprocs_runnin"..., 1200) = 1200
lseek(3, 0, SEEK_SET)                   = 0
read(3, "cpu  0 0 3375 498153 97 0 10 0 0"..., 4096) = 4096
read(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
read(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
read(3, " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0"..., 4096) = 4096
read(3, "5938\nprocesses 2774\nprocs_runnin"..., 4096) = 1200
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7f769284f000, 4096)            = 0
exit_group(0)                           = ?
+++ exited with 0 +++
[root@pro_app_server workspace]#


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