Sending exec output from function to main method - c

Question

I have a method I call from the main method called that executes ls-l on a certain directory, I want it to execute it and send the result as a string to the main method.
My current flawed code:
char *lsl(){
char *stringts=malloc(1024);
chdir("/Users/file/path");
char * lsargs[] = { "/bin/ls" , "-l", NULL};
stringts="The result of ls-l in the created directory is:"+ execv(lsargs[0], lsargs);
return stringts;
}
Currently I am only getting the exec output on the screen, I understand why this is happening(exec getting called before reaching return point). However I don't know how I could possibly do what I want and if it's actually doable.
I was thinking of using pipes and dup2() so I don't let the exec function use stdout but I don't know if it would be possible to put the output in a string.

Answer 1

As Jonathan Leffler already pointed out in comments, there is no '+' operator for concatenating strings in C.
A possibility to dynamically extends strings is to use realloc together with strcat.
For each number of bytes you read from the pipe, you could check the remaining capacity of the originally allocated memory for the string and, if this is not enough, reallocate twice the size.
You have to keep track of the size of the current string yourself. You could do this with a variable of type size_t.
If you combine this with the popen handling, it could look something like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
FILE *fp;
if ((fp = popen("ls -l", "r")) == NULL) {
perror("popen failed");
return EXIT_FAILURE;
}
size_t str_size = 1024;
char *stringts = malloc(str_size);
if (!stringts) {
perror("stringts allocation failed");
return EXIT_FAILURE;
}
stringts[0] = '\0';
char buf[128];
size_t n;
while ((n = fread(buf, 1, sizeof(buf) - 1, fp)) > 0) {
buf[n] = '\0';
size_t capacity = str_size - strlen(stringts) - 1;
while (n > capacity) {
str_size *= 2;
stringts = realloc(stringts, str_size);
if (!stringts) {
perror("stringts realloation failed");
return EXIT_FAILURE;
}
capacity = str_size - strlen(stringts) - 1;
}
strcat(stringts, buf);
}
printf("%s\n", stringts);
free(stringts);
if (pclose(fp) != 0) {
perror("pclose failed");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}

Answer 2

You have several flaws in your code:
char *lsl(){
char *stringts=malloc(1024);
chdir("/Users/file/path");
char * lsargs[] = { "/bin/ls" , "-l", NULL};
stringts="The result of ls-l in the created directory is:"+ execv(lsargs[0], lsargs);
return stringts;
}
If you malloc(3) a 1024 byte buffer into stringts pointer, but then you assign a different value to the pointer, making your buffer to be lost in the immensity of your RAM.
When you do execv(2) call, all the memory of your process is freed by the kernel and reloaded with an execution of the command ls -l, you'll get the output in the standard output of the process, and then you'll get the prompt of the shell. This makes the rest of your program unuseful, as once you exec, there's no way back, and your program is unloaded and freed.
You can add (+) to a pointer value (you indeed add to the address pointing to the string "The result of the ls -l..." and ---as the result of exec is nothing, as a new program is loaded--- you get nothing) If execv fails, then you get a pointer pointing to the previous char to that string, which is a valid expression in C, but makes your program to behave erratically in an Undefined Behaviour. Use strcpy(3), strcat(3), or snprintf(3), depending on the exact text you want to copy in the space of the buffer you allocated.
Your return an invalid address as a result. The problem here is that, if execv(2) works, it doesn't return. Only if it fails you get an invalid pointer that you cannot use (by the reason above), and of course ls -l has not been executed. Well, you don't say what you got as ouptut, so it is difficult for me to guess if you actually exec()d the program or not.
On other side, you have a popen(3) library function that allows you to execute a subprogram and allows you to read from a file descriptor its output (I recommend you not to chdir gratuitously in your program, as that is a global change in your program environment, IMHO it is better to pass ls(1) the directory you want to list as a parameter)
#include <stdio.h>
FILE *lsl() {
/* the call creates a FILE * descriptor that you can use as input and
* read the output of the ls command. It's bad resources use to try to
* read all in a string and return the string instead. Better read as
* much as you can/need and then pclose() the descriptor. */
return popen("/bin/ls -l /Users/file/path|", "rt");
}
and then you can read (as it can be very long output, you probably don't have enought buffer space to handle it all in memory if you have a huge directory)
FILE *dir = lsl();
if (dir) {
char buffer[1024];
while (fgets(buffer, sizeof buffer, dir)) {
process_line_of_lsl(buffer);
}
pclose(dir); /* you have to use pclose(3) with popen(3) */
}
If you don't want to use popen(3), then you cannot use execv(2) alone, and you have to fork(2) first, to create a new process, and exec() in the child process (after mounting the redirection yourself). Read a good introduction to fork()/exec() and how to redirect I/O between fork() and exec(), as it is far longer and detailed to put it here (again)