[英]C Redirecting stdout to a file for process called by exec in child process
I am unable to redirect STDOUT to a file. 我无法将STDOUT重定向到文件。
I have two processes. 我有两个过程。 These should run for 5 seconds and then be killed.
这些应运行5秒钟,然后杀死。
Process 1: redirects its STDOUT to a pipe and then prints random nubers. 流程1:将其STDOUT重定向到管道,然后打印随机数字。
Process 2: redirects its STDIN to the pipe and its STDOUT to file. 流程2:将其STDIN重定向到管道,并将其STDOUT重定向到文件。 Then it calls procedure appc1 by execl().
然后,它通过execl()调用过程appc1。 This procedure prints to STDOUT.
此过程将打印到STDOUT。
Problem: File out.txt contains the test print done by reader process, but nothing else from the appc1 procedure. 问题:文件out.txt包含读取器进程完成的测试打印,但是appc1过程中没有其他内容。 If I skip the redirection and print to console, everything works fine.
如果我跳过重定向并打印到控制台,一切正常。
Could it be an issue caused by killing the processes? 可能是由于终止进程引起的问题吗? Do I close the file too soon?
我是否过早关闭文件? Thank you very much for any advice, I am unable to solve this for quite some time.
非常感谢您的任何建议,我已经有一段时间无法解决此问题了。
ret = pipe(pipefd);
if (ret == -1) {
printf("Error creating pipe");
exit(1);
}
reader = fork();
if (reader == 0) {
dup2(pipefd[0], STDIN_FILENO);
file = fopen("out.txt", "w");
if(file == NULL) fputs("Could not open file",stderr);
dup2(fileno(file), STDOUT_FILENO);
fclose(file);
printf("Test values: %d %d\n\n", 234, 598);
execl("appc1","appc1", NULL);
}
else {
printf("PARENT: forking writer\n");
writer = fork();
if (writer == 0) {
sleep(1);
dup2(pipefd[1], STDOUT_FILENO);
while (1) {
num1 = rand() % 1000 + 1;
num2 = rand() % 1000 + 1;
printf("%d %d\n", num1, num2);
sleep(1);
}
}
else {
sleep(5);
kill(reader, SIGUSR1);
kill(writer, SIGUSR1);
wait(NULL);
exit(0);
}
}
This is the code. 这是代码。
Thank you guys. 感谢大伙们。
I just ran the following code based upon your code and it worked for me. 我只是根据您的代码运行了以下代码,它对我有用。 I would suggest that you may have a problem with running appc1 and you should add a printf() after the execl() that should never print unless the execl() fails to process properly.
我建议您可能在运行appc1时遇到问题,并且应该在execl()之后添加一个printf(),除非execl()无法正确处理,否则它永远都不会打印。
appc1.c appc1.c
#include <stdlib.h>
#include <stdio.h>
int main()
{
printf("Output from appc1\n" );
}
pipes.c 管
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
int main ()
{
int ret;
int num1, num2;
pid_t reader;
pid_t writer;
int pipefd[2];
FILE *file;
printf( "PARENT: creating pipe\n" );
ret = pipe( pipefd );
if ( ret == -1 ) {
printf( "Error creating pipe - %s", strerror( errno ) );
exit (1);
}
printf( "PARENT: forking reader\n" );
reader = fork();
if ( reader == -1 ) {
printf( "failed to fork reader\n" );
exit (1);
} else if ( reader == 0 ) {
printf( "READER CHILD\n" );
ret = dup2( pipefd[0], STDIN_FILENO );
if ( ret == -1 ) {
printf( "dup2 failed on pipefd[0] %s" , strerror( errno ) );
exit (1);
}
file = fopen( "out.txt", "w" );
if ( file == NULL ) {
fputs( "Could not open file", stderr );
exit (1);
}
ret = dup2( fileno( file ), STDOUT_FILENO );
if ( ret == -1 ) {
printf( "dup2 failed on fileno() %s" , strerror( errno ) );
exit (1);
}
fclose( file );
printf( "Test values: %d %d\n\n", 234, 598 );
execl( "appc1", "appc1", NULL );
printf( "execl failed - %s\n", strerror( errno ) );
} else {
printf( "PARENT: forking writer\n" );
writer = fork();
if ( writer == -1 ) {
printf( "Failed forking writer\n" );
exit (1);
} else if ( writer == 0 ) {
printf( "WRITER CHILD\n" );
sleep( 1 );
ret = dup2( pipefd[1], STDOUT_FILENO );
if ( ret == -1 ) {
printf( "dup2 failed on pipefd[1] %s" , strerror( errno ) );
exit (1);
}
while( 1 ) {
num1 = rand () % 1000 + 1;
num2 = rand () % 1000 + 1;
printf( "WRITER CHILD %d %d\n", num1, num2 );
sleep( 1 );
}
} else {
printf( "PARENT: sleeping\n" );
sleep( 5 );
printf( "PARENT: sending SIGUSR1 to reader\n" );
kill( reader, SIGUSR1 );
printf( "PARENT: sending SIGUSR1 to writer\n" );
kill( writer, SIGUSR1 );
wait( NULL );
exit( 0 );
}
}
}
console output: 控制台输出:
pipes
PARENT: creating pipe
PARENT: forking reader
PARENT: forking writer
READER CHILD
PARENT: sleeping
WRITER CHILD
PARENT: sending SIGUSR1 to reader
PARENT: sending SIGUSR1 to writer
out.txt content: out.txt内容:
Test values: 234 598
Output from appc1
When I changed appc1 execl() call to a appc1x (which does not exist) then out.txt contains: 当我将appc1 execl()调用更改为appc1x(不存在)时,out.txt包含:
Test values: 234 598
execl failed - No such file or directory
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.