Reading my child pipe created by exec() with C

Question

I'm just starting to learn C programming and I have some uncertainty about fork(), exec(), pipe(), etc.

I've developed this code, but when I execute it, the variable c remains empty, so I don't know if the child isn't writing to the pipe, or the parent isn't reading from it.

Could you help me please? This is the code:

int main() { int pid=0; int pipefd[2]; char* c=(char *)malloc(sizeof(char)); FILE *fp; pipe(pipefd); pid=fork(); if (pid==0){ close(pipefd[0]); dup2(pipefd[1],1); close(pipefd[1]); execl("ls -l | cut -c28","ls -l | cut -c28", (char *) 0); } else{ close(pipefd[1]); read(pipefd[0], c, 1); char* path="/home/random"; char* txt=".txt"; char* root=malloc(strlen(path) + strlen(txt) + sizeof(char)); strcpy(root,path); strcat(root,c); strcat(root,txt); close(pipefd[0]); fp=fopen(root,"w+"); (...) }

The problem is that the final root string its only "/home/random.txt" because there is nothing in the char c, and what I want is to open the file "/home/random(number stored in char c).txt".

Check that your execl call isn't returning - I don't think you can call execl like that. — Flexo - Save the data dump
– Flexo - Save the data dump ♦, Commented Sep 21, 2012 at 15:32

epsalon · Accepted Answer · 2012-09-21 15:32:41Z

1

execl executes a single command, and is not aware of shell concepts such as pipes. If you want to execute a shell command, you will have to execute a shell, as follows:

execl("/bin/sh","/bin/sh","-c","ls -l | cut -c28", (char*) 0);

answered Sep 21, 2012 at 15:32

epsalon

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2 Comments

Flexo - Save the data dump Over a year ago

Might be worth mentioning the NULL argv[0] - I don't think that's allowed either.

epsalon Over a year ago

He did pass an argv[0], the same as the program name.

Nikolai Fetissov · Accepted Answer · 2012-09-21 15:33:52Z

Always check the return value of the system calls (like execve(2) and derived functions like execl(3)), and use the errno(3) to figure out what went wrong.

In your case the execl line fails.

Paul Rubel · Accepted Answer · 2012-09-21 15:38:44Z

Using strcpy/strcat seems a bit excessively complex. snprintf can turn those 3 lines into one.

snprintf( root, size_of_buf, "/home/random%s", c );

Additionally, check your error codes. As noted, execl is failing and you don't know it. fork, dup2, ...,can also fail, you want to know sooner rather than later.

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