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。
- 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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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]#
- 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]#
用户 read() 和 fread() 检索/proc下的其他文件，看起来很正常
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]#
- 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]#