[英]Sending information with unnamed pipes
我的程序必須使用未命名的管道發送一些字節的信息。 我有一個名為“ input”的txt文件,應該由程序讀取,並且它的信息必須發送和寫入另一個名為“ output”的文件中。 我還必須使用read(),write(),open()函數。 我的代碼如下所示:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
#define BUFSIZE 25
int main( int argc, char *argv[] ) {
srand(time(NULL));
pid_t pid;
int mypipefd[2];
int ret;
char buf[BUFSIZE];
int output;
int stream;
int nbytes;
ret = pipe(mypipefd);
if( ret == -1 ) {
perror( "pipe error");
exit(1);
}
pid = fork();
if( pid == -1 ) {
perror( "FORK ERROR...");
exit(2);
}
if( pid == 0 ) {
/* CHILD */
printf(" Child process...\n");
stream = open("input.txt", O_RDONLY);
if (close(mypipefd[0]) == -1 ) {
perror("ERROR CLOSING PIPE");
exit(3);
}
while ( (nbytes = read(stream, buf, BUFSIZE)) > 0 ) {
sleep(rand() %2);
write(mypipefd[1], buf, nbytes );
}
if ( close(stream) == -1 ) {
perror("ERROR CLOSING STREAM");
exit(4);
}
}
else {
/* PARENT */
printf(" Parent process...\n");
output = open("output.txt", O_CREAT | O_WRONLY, 00777);
while ( (nbytes = read(mypipefd[0], buf, BUFSIZE)) > 0 ) {
write(output, buf, nbytes);
}
printf("buf: %s\n", buf);
if (close(output) == -1) {
perror("ERROR CLOSING OUTPUT");
exit(5);
}
if (close(mypipefd[1]) == -1 ) {
perror("ERROR CLOSING PIPE");
exit(6);
}
}
return 0;
}
不幸的是,代碼無法在終端屏幕上顯示
在我嘗試while循環並立即發送所有信息之前,它可以工作,但是輸出文件看起來像這個輸出文件
而輸入文件看起來像這個輸入文件
主要錯誤是父級必須在父級讀取循環之前 (而不是之后)執行close(mypipefd[1])
)。 這樣可以防止在孩子完成書寫后,父母在管道上看到EOF。
另外,您在父級中缺少一個waitpid
。
父級中buf
的printf
在錯誤的位置[在讀取循環之后]。 在這一點上,不能保證buf
擁有正確的數據或正確的零終止符。 這就是為什么stdout在末尾有一些垃圾字符。
因此,除了輸出到輸出文件之外,循環還應該輸出到stdout,但是應該使用fwrite
因為buf
不能保證被零終止。
我在最初的帖子中錯過了這一點,所以我已經更正了。
根據我的最高評價,孩子應該在[可能]部分寫入管道時循環執行。 我編碼了。
這是帶注解並修正錯誤的版本:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#define BUFSIZE 25
int main( int argc, char *argv[] ) {
srand(time(NULL));
pid_t pid;
int mypipefd[2];
int ret;
char buf[BUFSIZE];
int output;
int stream;
int nbytes;
ret = pipe(mypipefd);
if( ret == -1 ) {
perror( "pipe error");
exit(1);
}
pid = fork();
if( pid == -1 ) {
perror( "FORK ERROR...");
exit(2);
}
if( pid == 0 ) {
/* CHILD */
printf(" Child process...\n");
stream = open("input.txt", O_RDONLY);
if (close(mypipefd[0]) == -1 ) {
perror("ERROR CLOSING PIPE");
exit(3);
}
while ( (nbytes = read(stream, buf, BUFSIZE)) > 0 ) {
sleep(rand() %2);
#if 0
write(mypipefd[1], buf, nbytes );
#else
// NOTE: this _should_ work but adds extra at the end
int off;
int wlen;
for (off = 0; nbytes > 0; off += wlen, nbytes -= wlen) {
wlen = write(mypipefd[1], buf + off, nbytes );
if (wlen <= 0)
break;
}
#endif
}
if ( close(stream) == -1 ) {
perror("ERROR CLOSING STREAM");
exit(4);
}
// NOTE/FIX: child must close it's side of the pipe
#if 1
close(mypipefd[1]);
#endif
}
else {
/* PARENT */
printf(" Parent process...\n");
// NOTE/FIX: this must be closed _before_ the read loop -- holding it
// open prevents parent from seeing EOF on pipe
#if 1
if (close(mypipefd[1]) == -1 ) {
perror("ERROR CLOSING PIPE");
exit(6);
}
#endif
#if 1
printf("buf: ");
#endif
output = open("output.txt", O_CREAT | O_WRONLY, 00777);
while ( (nbytes = read(mypipefd[0], buf, BUFSIZE)) > 0 ) {
write(output, buf, nbytes);
#if 1
fwrite(buf,1,nbytes,stdout);
#endif
}
// NOTE/BUG: the buffer at this point will only have the data from
// the _last_ read and may not be null terminated
#if 0
printf("buf: %s\n", buf);
#else
printf("\n");
#endif
if (close(output) == -1) {
perror("ERROR CLOSING OUTPUT");
exit(5);
}
// NOTE/BUG: this must be closed _before_ the parent's read loop
#if 0
if (close(mypipefd[1]) == -1 ) {
perror("ERROR CLOSING PIPE");
exit(6);
}
#endif
// NOTE/FIX: this is missing (prevents orphan/zombie child process)
#if 1
waitpid(pid,NULL,0);
#endif
}
return 0;
}
更新:
但我不明白“ for”循環在這里做什么
寫入管道會產生“短寫入”(例如,您想寫入20,但返回值(即實際寫入的字節數))返回15。您必須索引緩沖區並在隨后的操作中寫入其余字節寫道。
內核對單個原子寫操作中可以寫多少個字節有一個限制(例如,如果您執行了write(mypipefd[1],buf,10000000)
,則內核沒有為如此大的寫操作分配空間,因此它將返回可以附加到管道緩沖區的值(在內核中)。
另外,假設內核管道緩沖區可以容納64個字節。 然后將大小為64的緩沖區寫入其中。 讀者可能只讀取32個字節。 因此,第一次寫就可以了。 然后,讀取器讀取32個字節。 因此,下一次寫入64的管道時,只有32個字節的空間,因此寫入將返回32
程序必須顯示:“ buf:這是ra”,然后顯示“ buf:ndom文本”
好吧,我已經解決了
最后,我需要在所有地方實現錯誤處理。
我注釋了要添加錯誤和處理的地方,以及需要查找的內容。
無論如何,這是更新版本。 我留在// NOTE/*
注釋中,但刪除了#if/#endif
對以使其更容易閱讀。
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#define BUFSIZE 25
int
main(int argc, char *argv[])
{
srand(time(NULL));
pid_t pid;
int mypipefd[2];
int ret;
char buf[BUFSIZE];
int output;
int stream;
int nbytes;
ret = pipe(mypipefd);
if (ret == -1) {
perror("pipe error");
exit(1);
}
pid = fork();
if (pid == -1) {
perror("FORK ERROR...");
exit(2);
}
if (pid == 0) {
/* CHILD */
printf(" Child process...\n");
stream = open("input.txt", O_RDONLY);
if (close(mypipefd[0]) == -1) {
perror("ERROR CLOSING PIPE");
exit(3);
}
while ((nbytes = read(stream, buf, BUFSIZE)) > 0) {
sleep(rand() % 2);
// NOTE/FIX: writing to pipes _can_ generate a _short_ write. that
// is, (e.g.) if the length given to write is 20, the return value
// may be only 15. this means that the remaining 5 bytes must be
// sent in a second/subsequent write
int off;
int wlen;
for (off = 0; nbytes > 0; off += wlen, nbytes -= wlen) {
wlen = write(mypipefd[1], buf + off, nbytes);
if (wlen < 0) {
perror("ERROR WRITING TO FILE");
exit(3);
}
if (wlen == 0)
break;
}
}
if (close(stream) == -1) {
perror("ERROR CLOSING STREAM");
exit(4);
}
// NOTE/FIX: child must close it's side of the pipe
// NOTE/ERRCODE: check error code here
close(mypipefd[1]);
}
else {
/* PARENT */
printf(" Parent process...\n");
// NOTE/FIX: this must be closed _before_ the read loop -- holding it
// open prevents parent from seeing EOF on pipe
if (close(mypipefd[1]) == -1) {
perror("ERROR CLOSING PIPE");
exit(6);
}
// NOTE/ERRCODE: this should be checked for -1 (i.e. output file
// could not be opened for file permission, etc. or other reasons
// similar to those for the file write below)
output = open("output.txt", O_CREAT | O_WRONLY, 00777);
// NOTE/FIX: we read one less than buffer size to allow for adding an
// artificial zero byte at the end
while ((nbytes = read(mypipefd[0], buf, BUFSIZE - 1)) > 0) {
// NOTE/ERRCODE: error handling _could_ be added here but it would
// be rare (e.g. filesystem has an I/O error because it's full or
// marked R/O because of an I/O error on the underlying disk)
write(output, buf, nbytes);
// write partial buffer to stdout
buf[nbytes] = 0;
printf("buf: %s\n",buf);
}
if (close(output) == -1) {
perror("ERROR CLOSING OUTPUT");
exit(5);
}
// NOTE/FIX: this is missing (prevents orphan/zombie child process)
// NOTE/ERRCODE: yes, this _can_ have an error return but here it's
// unlikely because we _know_ that pid is valid
// what can be done is to do:
// int status;
// waitpid(pid,&status,0)
// then process the return code from the child using the W* macros
// provided (e.g. WIFEXITED, WSTATUS) on status
waitpid(pid, NULL, 0);
}
return 0;
}
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