c - 如何将程序输出重定向到文本文件

Question

我想将程序的输出重定向到一个文件。我怎样才能做到这一点？目前我的文件没有创建，我只能将输出打印到我的控制台。

    int fd[2];
    int processId;
    int output;
    char filename[] = "output.txt";

    if((output = open(filename, O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR)) == -1){
        fprintf(stderr, "Unable to create/open file '%s'\n", filename);
        return 1;
    }

    if(pipe(fd) == -1){
        fprintf(stderr, "Error creating pipe\n");
        return 2;
    }

    if((processId = fork()) == -1){
        fprintf(stderr, "Error forking\n");
        return 3;
    }   

    if(processId == 0){
        int newFD = dup(STDOUT_FILENO);
        char newFileDescriptor[2];
        sprintf(newFileDescriptor, "%d", newFD);
        dup2(fd[1], output);
        close(fd[0]);
        execl("./pipetest", "pipetest", newFileDescriptor, NULL);
    }else{
        close(fd[1]);
        char c[10];
        int r = read(fd[0], c, sizeof(char) * 10);

        if(r > 0){
            fprintf(stderr, "PIPE INPUT = %s", c);
            fwrite(c, 1, sizeof(c), output);
        }
    }

Answer 1

一个好的开始是不要忽略编译器警告:

test.c: In function ‘main’:
test.c:42:13: warning: passing argument 4 of ‘fwrite’ makes pointer from integer without a cast [enabled by default]
             fwrite(c, 1, sizeof(c), output);
             ^
In file included from test.c:1:0:
/usr/include/stdio.h:715:15: note: expected ‘struct FILE * __restrict__’ but argument is of type ‘int’
 extern size_t fwrite (const void *__restrict __ptr, size_t __size,
               ^

int 和 FILE* 不可互换。如果您使用open，请使用write 写入。如果您使用 fopen，请使用 fwrite 写入。

此外，您永远不会修改进程的标准输出。相反，您修改了 output，这没有任何意义。以下是为使其正常工作而对代码所做的最少更改:

#include <stdio.h>                                                                                                                                                                                                                                                                                                                                                                                                                       
#include <sys/types.h>                                                                                                                                                                                                                                                                                                                                                                                                                   
#include <sys/stat.h>                                                                                                                                                                                                                                                                                                                                                                                                                    
#include <fcntl.h>                                                                                                                                                                                                                                                                                                                                                                                                                       
#include <unistd.h>                                                                                                                                                                                                                                                                                                                                                                                                                      

int main() {                                                                                                                                                                                                                                                                                                                                                                                                                             
    int fd[2];                                                                                                                                                                                                                                                                                                                                                                                                                           
    int processId;                                                                                                                                                                                                                                                                                                                                                                                                                       
    int output;                                                                                                                                                                                                                                                                                                                                                                                                                          
    char filename[] = "output.txt";                                                                                                                                                                                                                                                                                                                                                                                                      

    if((output = open(filename, O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR)) == -1){                                                                                                                                                                                                                                                                                                                                                          
        fprintf(stderr, "Unable to create/open file '%s'\n", filename);                                                                                                                                                                                                                                                                                                                                                                  
        return 1;                                                                                                                                                                                                                                                                                                                                                                                                                        
    }                                                                                                                                                                                                                                                                                                                                                                                                                                    

    if(pipe(fd) == -1){                                                                                                                                                                                                                                                                                                                                                                                                                  
        fprintf(stderr, "Error creating pipe\n");                                                                                                                                                                                                                                                                                                                                                                                        
        return 2;                                                                                                                                                                                                                                                                                                                                                                                                                        
    }                                                                                                                                                                                                                                                                                                                                                                                                                                    

    if((processId = fork()) == -1){                                                                                                                                                                                                                                                                                                                                                                                                      
        fprintf(stderr, "Error forking\n");                                                                                                                                                                                                                                                                                                                                                                                              
        return 3;                                                                                                                                                                                                                                                                                                                                                                                                                        
    }                                                                                                                                                                                                                                                                                                                                                                                                                                    

    if(processId == 0){                                                                                                                                                                                                                                                                                                                                                                                                                  
        int newFD = dup(STDOUT_FILENO);                                                                                                                                                                                                                                                                                                                                                                                                  
        char newFileDescriptor[2];                                                                                                                                                                                                                                                                                                                                                                                                       
        sprintf(newFileDescriptor, "%d", newFD);                                                                                                                                                                                                                                                                                                                                                                                         
        dup2(fd[1], STDOUT_FILENO); // You want to modify STDOUT_FILENO                                                                                                                                                                                                                                                                                                                                                                  
        close(fd[0]);                                                                                                                                                                                                                                                                                                                                                                                                                    
        execl("/bin/echo", "echo", newFileDescriptor, NULL); // switched to echo                                                                                                                                                                                                                                                                                                                                                         
    }else{                                                                                                                                                                                                                                                                                                                                                                                                                               
        close(fd[1]);                                                                                                                                                                                                                                                                                                                                                                                                                    
        char c[10];                                                                                                                                                                                                                                                                                                                                                                                                                      
        int r = read(fd[0], c, sizeof(char) * 10);                                                                                                                                                                                                                                                                                                                                                                                       

        if(r > 0){                                                                                                                                                                                                                                                                                                                                                                                                                       
            fprintf(stderr, "PIPE INPUT = %s", c);                                                                                                                                                                                                                                                                                                                                                                                       
            write(output, c, r); // use write instead of fwrite                                                                                                                                                                                                                                                                                                                                                                          
        }                                                                                                                                                                                                                                                                                                                                                                                                                                
    }                                                                                                                                                                                                                                                                                                                                                                                                                                    
}                                                                                                                                                                                                                                                                                                                                                                                                                                        

运行结果如下:

$ gcc test.c -o test
$ ./test
PIPE INPUT = 6
$ cat output.txt 
6