Why is snprintf not reading after %s?

Question

I was trying to write a program that checks out if a location exists under the /home/user directory. To do that, I had to get the username with the whoami command and add the output of it to the buffer to use the locate command.

However, even though the snprintf read the whoami command, it didn't read the rest. I made a couple of searches and came to a result that NULL may not be terminated at the end of the string. Nevertheless, I couldn't find out how to terminate it manually. I am not sure what the problem is, so, here I am.

Here is the code for a better demonstration of my issue:

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>
#include <string.h>
    
char *readFile(char *);
bool check();
    
bool check() {
    char path[200] = { 0 };
    
    snprintf(path, 200, "/home/%s/.example", readFile("whoami"));
    
    char lll[300] = { 0 };
    
    snprintf(lll, 300, "locate %s", path);
    
    char *buffer = readFile(lll);
    
    if (strcmp(buffer, path) == 0) {
        return true;
    } else {
        return false;
    }
}

char *readFile(char cmd[200]) {
    char cmd1[99999] = { 0 };
    system("touch cmd");  
    snprintf(cmd1, 99999, "%s >> cmd", cmd);
    system(cmd1); 
        
    FILE *f = fopen("cmd", "rt");
    assert(f);
    fseek(f, 0, SEEK_END);
    long length = ftell(f);
    fseek(f, 0, SEEK_SET);
    char *buffer = (char *)malloc(length + 1);
    buffer[length] = '\0';
    fread(buffer, 1, length, f);
    fclose(f);
    system("rm cmd");
    return buffer;
}
    
int main() {
    int x = check();
    
    if (x == 1)
        printf("There is a location like that");
    else
        printf("There isn't");
    
    return 0;
}

Answer 1

The whoami command prints out the name of the current user, termintated by a newline. Reading back the file cmd into buffer will include that same newline character.

The path string will then be "/home/USERNAME\n/.example". Trying to execute locate /home/USERNAME\n/.example will probably confuse the system function.

The solution should then be to strip away the last newline in readFile.

Answer 2

There are some problems in your readFile function:

• the name is misleading, it should be runCommand or something equivalent.

• you append the command output to the cmd file. This will cause unexpected results if the cmd file exists before you run your program. You should instead write:

  snprintf(cmd1, 99999, "%s > cmd", cmd);
  

• the command output probably has a trailing newline. You should trim the output.

Here is a modified version:

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

char *readFile(const char *cmd) {
    /* construct the command */
    size_t size = 4 + strlen(cmd) + 1;
    char *cmd1 = malloc(size);
    assert(cmd1);
    snprintf(cmd1, size, "%s > cmd", cmd);

    /* run the command */
    system(cmd1);
    free(cmd1);
        
    /* read the output file */
    FILE *f = fopen("cmd", "rt");
    assert(f);
    fseek(f, 0, SEEK_END);
    long length = ftell(f);
    fseek(f, 0, SEEK_SET);
    char *buffer = malloc(length + 1);
    buffer[length] = '\0';
    fread(buffer, 1, length, f);
    fclose(f);
    unlink("cmd");

    /* trim trailing white space from the output */
    while (length > 0 && isspace((unsigned char)buffer[length - 1])) {
        buffer[--length] = '\0';
    }
    /* trim leading white space from the output */
    size_t i = 0;
    while (isspace((unsigned char)buffer[i]) {
        i++;
    }
    if (i > 0) {
        memmove(buffer, buffer + i, length - i + 1);
    }
    return buffer;
}

Answer 3

Since Linux has a shortcut for your home directory ("~"), the whole "whoami"/"getuid" mess can be avoided by just using:

char *buffer = readFile("locate ~/.example");

You will still potentially have to trim off trailing newline and/or CR characters, perhaps with:

strtok(buffer,"\r\n\t ");

Or better yet, as @chqrlie suggests below.