这个程序应该做的是询问用户一个简单的算术问题，例如： 5 + 7然后用“bc”检查答案（是否正确）。
我有以下代码，但我不明白如何编辑它以将结果从“5 + 7”存储到变量中（目前结果进入STDOUT）。
欢迎任何帮助。
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
int main( int argc, char *argv[], char *env[] )
{
char *expr = "5 + 7\n";
int answer;
printf("%s = ", expr);
scanf("%d", &answer);
int pfds[2];
pipe(pfds);
if (!fork()) {
close(1); /* close normal stdout */
dup(pfds[1]); /* make stdout same as pfds[1] */
close(pfds[0]); /* we don't need this */
printf("%s\n", expr);
/***********************/
/* How to store printf()'s output into a variable? */
exit(0);
} else {
close(0); /* close normal stdin */
dup(pfds[0]); /* make stdin same as pfds[0] */
close(pfds[1]); /* we don't need this */
execlp("bc", "bc", NULL);
}
return 0;
}
答案 0 :(得分：1)
您需要创建第二个管道并在子进程中将stdout重定向到它。
答案 1 :(得分：0)
您可以简单地读取STOUD或读取管道的输出。然后拨打Read()和Atoi可以完成工作。 Atoi手册页here
我不能为你编码，但这里是逻辑
`int fds[2];
pipe(fds);
dup2(fds[1], stdout);
read(fds[1], buf, buf_sz);
int ResultOfbc = Atoi(buf)`
答案 2 :(得分：0)
Ooof oof，我记得去年做的这些东西。 我基本上是一个使用自己的telnet克隆与互联网通信的程序。 问题是telnet使用stdin和stdout工作。
看起来你有相同的情况！ 我解决这个问题的方法是分叉一个进程，抓取stdin和stdout并将它们放入管道，然后通过调用telnet（现在代替stdin和stdout，使用这些管道）来覆盖分叉的进程映像。
您需要一个管道将文本发送到bc，而另一个管道则从bc接收。 如果您使用单个管道处理所有内容，您很可能最终会阅读您发送给bc并混合数据的内容。
警告：大量的代码收到。 我确定你不需要理解所有东西，因为我使用线程同时写入和读取并选择（）以查看管道中是否有任何要读取的内容。 很重要！！！当通信中断时，您的进程将收到SIGPIPE，它不会干净地终止（如果您正在使用动态内存或类似的东西）。 你必须fflush（outpipe）或否则bc将不会收到它。 （这是因为系统只有在找到'\ n'或类似的东西时才会刷新，如果我没记错的话）。 我把所有代码都放在以防你想要读取X的内容。但你需要的只是在“LOCAL FUNCTIONS END HERE”评论之后的小分叉
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>
#include <signal.h>
#include <stdbool.h>
#include "irc.h"
#include "coloring.h"
#include "rtsp.h"
#define BUFFERSIZE 255
int main (int argc, char *argv[]) {
/* XXX: When kid dies, program doesn't exit */
char *serverName, *port, *nick, *channel;
int ptelnetin[2];
int ptelnetout[2];
FILE *fpipes[2];
bool running = true;
pid_t kid;
pthread_t pthread_input, pthread_output;
/************************************************
LOCAL FUNCTIONS START HERE
***********************************************/
void *inputprocess(void *pipes) {
bool bracket;
int i;
fd_set rfds;
struct timeval tv;
tv.tv_sec = 0.2;
tv.tv_usec = 0;
char buffer[BUFFERSIZE];
FILE *out = ((FILE **) pipes)[1];
while (running){
FD_ZERO(&rfds);
FD_SET(fileno(stdin), &rfds);
switch (select(FD_SETSIZE, &rfds, NULL, NULL, &tv)) {
case -1:
fprintf(stderr, "Error reading data at select() Line %d, File %s\n", __LINE__, __FILE__);
running = false;
break;
case 0:
/* There's no data avaiable just yet.
Do nothing and keep checking */
break;
default:
/* This check needs to be done;
select isn't completely reliable */
if(!fgets(buffer, BUFFERSIZE, stdin)) {
running = false;
break;
}
/* Check message not to contain brackets*/
for (i = 0, bracket = false; running && !bracket && buffer[i] && i < BUFFERSIZE; i++) {
if (buffer[i] == '[' || buffer[i] == ']') {
PRINT_YELLOW;
printf("Use of brackets not allowed\n");
RESET_COLOR;
fflush(stdout);
bracket = true;
break;
}
}
if (running && !bracket) ircInputWrite(out, buffer);
fflush(out);
}
}
}
void *outputprocess(void *pipes){
fd_set rfds;
struct timeval tv;
tv.tv_sec = 0.2;
tv.tv_usec = 0;
char buffer[BUFFERSIZE];
char from[100];
FILE *in = ((FILE **) pipes)[0];
FILE *out = ((FILE **) pipes)[1];
while (running){
FD_ZERO(&rfds);
FD_SET(fileno(in), &rfds);
switch (select(FD_SETSIZE, &rfds, NULL, NULL, &tv)) {
case -1:
fprintf(stderr, "Error reading data at select() Line %d, File %s\n", __LINE__, __FILE__);
running = false;
break;
case 0:
/* There's no data avaiable just yet. */
/* Select sometimes returns 0 when there IS
data to read so we'll read anyway */
default:
/* This check needs to be done;
select isn't completely reliable */
if(!fgets(buffer, BUFFERSIZE, in)) {
running = false;
break;
}
switch(ircWhatsthis(buffer)) {
case iPING:
PRINT_BLUE;
printf("PingPong!\n");
RESET_COLOR;
ircPingPong(out, buffer); fflush(out);
fflush(stdout);
break;
case iROOMMSG:
if (ircUnpackPRIVMSG(from, buffer, buffer)) {
PRINT_BRIGHT_RED;
fprintf(stdout, "Malformed private message received\n");
RESET_COLOR;
}
else {
PRINT_CYAN;
printf("<%s>: ", from);
puts(buffer);
RESET_COLOR;
}
fflush(stdout);
break;
case iPRIVMSG:
fflush(stdout);
if (ircUnpackPRIVMSG(from, buffer, buffer)) {
PRINT_BRIGHT_RED;
fprintf(stdout, "Malformed private message received\n");
RESET_COLOR;
fflush(stdout);
}
else {
if (rtspExecBrackets(out, from, buffer)) {
PRINT_BRIGHT_MAGENTA;
printf("[%s]: ", from);
puts(buffer);
RESET_COLOR;
fflush(stdout);
}
}
break;
case iERROR:
PRINT_BRIGHT_RED;
fwrite(buffer, sizeof(char), strlen(buffer)+1, stdout);
RESET_COLOR;
break;
case iOK:
PRINT_BRIGHT_CYAN;
fwrite(buffer, sizeof(char), strlen(buffer)+1, stdout);
RESET_COLOR;
break;
default:
PRINT_BRIGHT_BLACK;
fwrite(buffer, sizeof(char), strlen(buffer)+1, stdout);
RESET_COLOR;
}
fflush(stdout);
}
}
}
void terminate(int signum) {
/* XXX irc.c calls rtsp.c which uses threads.
These threads never clean up if exiting via ^C
*/
RESET_COLOR;
running = false;
/*Close IO*/
fclose(fpipes[0]);
fclose(fpipes[1]);
/* Call child */
kill(kid, SIGINT);
wait(NULL);
exit(0);
}
/************************************************
LOCAL FUNCTIONS END HERE
***********************************************/
signal(SIGPIPE, terminate);
signal(SIGINT, terminate);
/* Get parameters */
if (argc != 5) {
fprintf(stderr, "Usage:\n %s <server> <port> <nick> <channel>\n", argv[0]);
return -1;
}
serverName = argv[1];
port = argv[2];
nick = argv[3];
channel = argv[4];
/* Startup pipes */
pipe(ptelnetin);
pipe(ptelnetout);
/* Launch telnete */
switch (kid = fork()) {
case -1:
perror("OMG ABORTION at main");
exit(-2);
case 0: /* CHILD */
/*Overwrite stdin with pipein and discard pipe*/
dup2(ptelnetin[0], 0);
close(ptelnetin[0]);
close(ptelnetin[1]);
/*Overwrite stdout with pipeout and discard pipe*/
dup2(ptelnetout[1], 1);
close(ptelnetout[0]);
close(ptelnetout[1]);
/*Overwrite process image with telnete*/
execlp("./telnete", "./telnete", argv[1], argv[2], (char *) NULL);
perror("Call to exec failed at main");
exit(-3);
default: /* PARENT */
/* Close reading end of pipein */
close(ptelnetin[0]);
/* Close writing end on pipeout */
close(ptelnetout[1]);
}
/* Turn (fileno) into (FILE *) */
fpipes[1] = fdopen(ptelnetin[1],"w");
if(!fpipes[1]) {
perror("Error at fdopen(in) at main");
kill(kid, SIGINT);
abort();
}
fpipes[0] = fdopen(ptelnetout[0],"r");
if(!fpipes[0]) {
perror("Error at fdopen(out) at main");
kill(kid, SIGINT);
abort();
}
/* Sleep for a few seconds so server doesn't ignore it */
PRINT_YELLOW;
printf("Logging in IRC...\n");
RESET_COLOR;
fflush(stdout);
if (ircRegister(argv[3], fpipes[1], fpipes[0])) {
fprintf(stderr, "Error registering in IRC.\n");
terminate(-1);
}
PRINT_YELLOW;
printf("Joining room %s\n", argv[4]);
RESET_COLOR;
ircJOIN(fpipes[1], argv[4]);
fflush(fpipes[1]);
/* Launch threads */
if (pthread_create(&pthread_input, NULL, inputprocess, fpipes)){
fprintf(stderr,"Couldn't launch input thread");
kill(kid, SIGINT);
abort();
}
if (pthread_create(&pthread_output, NULL, outputprocess, fpipes)){
fprintf(stderr,"Couldn't launch output thread");
kill(kid, SIGINT);
abort();
}
/* Wait for threads */
if (pthread_join(pthread_input,NULL)){
fprintf(stderr, "Error joining thread.\n");
}
if (pthread_join(pthread_output,NULL)){
fprintf(stderr,"Error joining thread.\n");
}
terminate(0);
}
我会将关键片段放在这里，以便更清楚：
/* Startup pipes */
pipe(ptelnetin);
pipe(ptelnetout);
/* Launch telnete */
switch (kid = fork()) {
case -1:
perror("OMG ABORTION at main");
exit(-2);
case 0: /* CHILD */
/*Overwrite stdin with pipein and discard pipe*/
dup2(ptelnetin[0], 0);
close(ptelnetin[0]);
close(ptelnetin[1]);
/*Overwrite stdout with pipeout and discard pipe*/
dup2(ptelnetout[1], 1);
close(ptelnetout[0]);
close(ptelnetout[1]);
/*Overwrite process image with telnete*/
execlp("./telnete", "./telnete", argv[1], argv[2], (char *) NULL);
perror("Call to exec failed at main");
exit(-3);
default: /* PARENT */
/* Close reading end of pipein */
close(ptelnetin[0]);
/* Close writing end on pipeout */
close(ptelnetout[1]);
}
/* Turn (fileno) into (FILE *) */
fpipes[1] = fdopen(ptelnetin[1],"w");
if(!fpipes[1]) {
perror("Error at fdopen(in) at main");
kill(kid, SIGINT);
abort();
}
fpipes[0] = fdopen(ptelnetout[0],"r");
if(!fpipes[0]) {
perror("Error at fdopen(out) at main");
kill(kid, SIGINT);
abort();
}
执行此操作后，您可以从（FILE *）fpipes [0]中读取bc的结果，并在fpipes [1]中写入。 记得在每次写入后fflush（fpipes [1]）。 像对待任何文件一样对待这两个。