我有一份任務正在處理中,我很難完成它。這個想法是編寫一個程序if.c執行一個程序,如果成功執行第二個程序。我應該壓制第一個程序的標準輸出和第二個非壓縮標準輸出。我在多個測試中收到錯誤消息。例如:「./if echo no then echo yes」returns「echo:write error:Bad file descriptor」。我試圖找到我在網上做錯了,但沒有運氣。在C中,如何使用dup2將STDOUT_FILENO重定向到/ dev/null,然後再重定向回它的原始值?
這裏是我的代碼:
#include <fcntl.h>
#include <sys/wait.h>
#include <stdio.h>
#include "tlpi_hdr.h"
int main(int argc, char *argv[])
{
if(argc < 4){
fprintf(stderr,"Incorrect number of arguments.\n");
exit(EXIT_FAILURE);
}
int thenArg = 0;
char then[4];
strcpy(then,"then");
for(int x=1; x<argc; x++){
if(strncmp(argv[x], then, 4) == 0) thenArg = x;
}
if(thenArg == 0){
fprintf(stderr,"No 'then' argument found.\n");
exit(EXIT_FAILURE);
}
int save_out = dup(STDOUT_FILENO);
if(save_out == -1){
fprintf(stderr,"Error in dup(STDOUT_FILENO)\n");
exit(EXIT_FAILURE);
}
int devNull = open("/dev/null",0);
if(devNull == -1){
fprintf(stderr,"Error in open('/dev/null',0)\n");
exit(EXIT_FAILURE);
}
int dup2Result = dup2(devNull, STDOUT_FILENO);
if(dup2Result == -1) {
fprintf(stderr,"Error in dup2(devNull, STDOUT_FILENO)\n");
exit(EXIT_FAILURE);
}
int program1argLocation = 1;
int program2argLocation = thenArg + 1;
int program1argCount = thenArg-1;
int program2argCount = argc-(program2argLocation);
char *program1args[program1argCount+1];
char *program2args[program2argCount+1];
for(int i=0; i<program1argCount; i++){
program1args[i]=argv[program1argLocation + i];
}
program1args[program1argCount] = NULL;
for(int i=0; i<program2argCount; i++){
program2args[i]=argv[program2argLocation + i];
}
program2args[program2argCount] = NULL;
pid_t pid = fork();
int child_status;
switch (pid) {
case -1:
fprintf(stderr,"Fork failed\n");
exit(EXIT_FAILURE);
case 0: //child
//child will run program 1
if(execvp(program1args[0],&program1args[0]) == -1){
fprintf(stderr,"Program 1 Failed.\n");
exit(EXIT_FAILURE);
}
default: //parent
//parent will run program2
pid = wait(&child_status);
if(WEXITSTATUS(child_status) == 0){
dup2(save_out, STDOUT_FILENO);
int prog2status = execvp(program2args[0],&program2args[0]);
if(prog2status == -1) {
fprintf(stderr,"Program 2 failed.\n");
exit(EXIT_FAILURE);
}
}
}
}
'字符,然後[ 4]; strcpy(然後,「然後」);'是一個緩衝區溢出。你爲什麼不用'if(0 == strcmp(argv [x],then「))'或'if(0 == strncmp(argv [x],」then「,4))'而不是? – nneonneo 2013-02-13 05:16:44
我想我第一次實現它並遇到某種錯誤。我會記住這一點,並在下次遇到同樣的事情時再次參考它。謝謝!還不習慣C,所以像這樣的任何輸入都很棒。 – Frank 2013-02-13 05:28:39