How strsep works in C?

Question

I'm creating a program that when launched takes in input a command and some arguments with scanf and calls execvp using those arguments. I'm doing this with strsep. I store the string in an array (char*),then I want to split it and store tokens in a new array (this time it's an array[] so I can use it with the execvp). Arguments saved with scanf should be commands for terminal (as "ls" with "-l" ecc,"pwd"... however variables saved in PATH) so they are separated by " ".

Ex : 

./mystring  
Type arguments : " ls -l "

It was an example only to specify which kind of input 'll be. I'll do the execvp alone,I need help to split the string in tokens. This is the code :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
 fflush(NULL); //to clean streams
 printf("type the command to start (and arguments if required) \n");
 char **st;
 char dest[10];
 scanf("%s",*st);
 int i;
 for (i=0;(dest[i]=strsep(st," "))!=NULL;i++)
    continue;
 for (int c=0;c<i;c++) printf(" arg %d : [%s] ",c,dest[c]);
 return 0;
}

Lines 5 and 6 required to call strsep,the 10 in dest[10] is symbolic.

Line 7 to store the input in st.

Line 9 should split on " " and store command and arguments in dest[I](that I'll pass to execvp).

Line 11 to print what dest has stored.

And this is the sad output :

./mystring 
type the command to start (and arguments if required) 
Segmentation fault: 11

I don't understand how strsep works, someone can help me?

Answer 1

You need to pass strsep() a pointer to the string to be analyzed. That means something more like:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    printf("type the command to start (and arguments if required)\n");
    char *dest[10];
    char line[4096];
    if (fgets(line, sizeof(line), stdin) == 0)
        return 1;
    line[strcspn(line, "\n")] = '\0';  /* Zap trailing newline */
    char *st = line;
    int i;
    for (i = 0; i < 10 && (dest[i] = strsep(&st, " ")) != NULL; i++)
        ;
    for (int c = 0; c < i; c++)
        printf(" arg %d : [%s]\n", c, dest[c]);
    return 0;
}

Note the changes to dest and st — and the addition of spaces to make the code more easily readable (and newlines in the output). I removed the odd and irrelevant fflush(NULL), too — when a program starts, the buffers don't need flushing. The code removes the newline from the input; scanf() would not have included any newline. The notation scanf("%s", *st); in the question would only read one word from the input, even assuming that *st pointed to some allocated memory (which, in the question, it doesn't).

Note that description of strsep() from a macOS man page says:

char *strsep(char **stringp, const char *delim);

The strsep() function locates, in the string referenced by *stringp, the first occurrence of any character in the string delim (or the terminating '\0' character) and replaces it with a '\0'. The location of the next character after the delimiter character (or NULL, if the end of the string was reached) is stored in *stringp. The original value of *stringp is returned.

This implies that you need to pass a pointer to a char * variable, rather than the value of a char ** variable. The macOS man page also includes an example which shows similar code, passing &string to strsep() where char *string = …;. There is sometimes an art to reading a man page — and reading between the lines of the man page.

The code above (now tested) can be run to produce:

$ ./ss53
type the command to start (and arguments if required)
command-name-is-long a short arg
 arg 0 : [command-name-is-long]
 arg 1 : [a]
 arg 2 : [short]
 arg 3 : [arg]
$