Segmentation fault while system() in C - c

Question

I want to use the "base64" script of linux to encode the data and get it in C.
When I try to compile
char a[200];
strcpy(a, "Hello");
printf("%s", a);
I get the output
Hello
Now whenever I try the code
char a[200];
strcpy(a, system("echo Hello | base64"));
printf("%s", a);
I get the output
aGVsbG8K
Segmentation fault
Even when I remove the "printf" statement, I get the same
aGVsbG8K
Segmentation fault
I want to save the value of the output of
system("echo Hello | base64")
in 'a' and not display it. Please help

Answer 1

If you read the documentation for system you'll discover that it doesn't return a string - it's defined as:
int system(const char *command);
The return value is the return status of the command or -1 if there's an error. You can't get the output using system - the output of the command(s) you run will go straight to stdout.
To get the output from another command you could use something like popen.
FILE *myfile;
char buffer[1024];
myfile=popen("echo Hello | base64","r");
if(myfile)
{
while(fgets(buffer,1024,myfile))
{
printf("%s",buffer);
}
pclose(myfile);
}

Answer 2

Here
strcpy(a, system("echo Hello | base64"));
system() doesn't stores it's result into array a as system() job is to execute the command provided in the argument & print it on console i.e stdout buffer. From the manual page of system
system() executes a command specified in command by calling
/bin/sh -c
command, and returns after the command has been completed.
There is one way to solve the problem i.e instead of printing system() output on stdout you can redirect its output to a file & then read that from file & print. For example
int main(void) {
close(1); /* stdout file descriptor is avilable now */
/* create the file if doesn't exist, if exist truncate the content to 0 length */
int fd = open("data.txt",O_CREAT|O_TRUNC|O_RDWR,0664); /* fd gets assigned with lowest
available fd i.e 1 i.e nowonwards stdout output
gets rediredcted to file */
if(fd == -1) {
/* #TODO error handling */
return 0;
}
system("echo Hello | base64"); /* system output gets stored in file */
int max_char = lseek(fd,0,2);/* make fd to point to end, get the max no of char */
char *a = malloc(max_char + 1); /* to avoid buffer overflow or
underflow, allocate memory only equal to the max no of char in file */
if(a == NULL) {
/* #TODO error handling if malloc fails */
return 0;
}
lseek(fd,0,0);/* from beginning of file */
int ret = read(fd,a,max_char);/* now read out put of system() from
file as array and print it */
if(ret == -1) {
/* #TODO error handling */
return 0;
}
a[ret] = '\0';/* \0 terminated array */
dup2(0,fd);/*fd 0 duplicated to file descriptor where fd points i.e */
printf("output : %s \n", a);
/* to avoid memory leak, free the dynamic memory */
free(a);
return 0;
}
My above suggestion is a temporary fix & I won't recommend this, instead use [popen] as suggested by #chris Turner (http://man7.org/linux/man-pages/man3/popen.3.html) which says
The popen() function opens a process by creating a pipe, forking,
and
invoking the shell. Since a pipe is by definition unidirectional, the
type argument may specify only reading or writing, not both; the
resulting stream is correspondingly read-only or write-only.
For example
int main(void) {
char buf[1024];
FILE *fp = popen("echo Hello | base64","r");
printf("%s\n",fgets(buf,sizeof(buf),fp));
return 0;
}