Modeling With Data

Tip 10: Use asprintf to make string handling less painful

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Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

level: basic string user
purpose: don't call malloc

The function allocates the amount of string space you will need, and then fills the string. That means you never really have to worry about string-allocing again.

asprintf is not part of the C standard, but it's available on systems with the GNU or BSD standard library, which covers pretty much everybody (including Apple computers).

The old way made people homicidal (or suicidal, depending on temprament) because you first have to get the length of the string you are about to fill, allocate space, and then actually write to the space. ¡Don't forget the extra slot in the string for the null terminator!

This sample program demonstrates the painful way of setting up a string, for the purpose of using C's system command to run an external program. The thematically appropriate program, strings, searches a binary for printable plain text. I'm assuming that you're compiling to a.out, in which case the program searches itself for strings. This is perhaps amusing, which is all we can ask of demo code.

#include <stdio.h>
void get_strings(char *in){
    char *cmd;
    int len = strlen("strings ") + strlen(in) + 1;
    cmd = malloc(len); //The C standard says sizeof(char)==1, b.t.w.
    snprintf(cmd, len, "strings %s", in);
    system(cmd);
}

int main(){
    get_strings("a.out");
}

Apophenia users, use apop_system("strings %s", in) to bypass the need to first print to a string and then call system. Everybody else, feel free to write your own system_with_printf function; it's not that hard.

With asprintf, malloc gets called for you, which means that you also don't need the step where you measure the length of the string:

#include <stdio.h>
void get_strings(char *in){
    char *cmd;
    asprintf(&cmd, "strings %s", in);
    system(cmd);
}

int main(){
    get_strings("a.out");
}

The actual call to asprintf looks a lot like the call to sprintf, except you need to send the location of the string, not the string itself, because something new will be malloced into that location. Also, we don't need to send the length parameter like with snprintf, because asprintf is smart enough to count letters for you.

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