Question

I am making a C program. I have a function that everything seems to be working great. But if I add another variable to it it suddenly does not work and I get a segmentation fault. (If I use gdb I get the error: Single stepping until exit from function __svfscanf_l,
which has no line number information. Program received signal: “EXC_BAD_ACCESS”.)

Here is my function:

void condition(char *command) {
printf("CAMMAND: %s\n", command);
char *cond;
char *expression;
char *file;
char *buffer;
buffer = malloc(200 * sizeof(char));

sscanf(command, "%s ( %s %s ) %[^\n]", cond, expression, file, buffer);

printf("COND: %s\n", cond);
printf("EX: %s\n", expression);
printf("File: %s\n", file);
printf("Command: %s\n", buffer);

if (!strcmp(expression, "checke")) {
    printf("CHECK EXISTS\n");
    if (file_exists(file)) {
        printf("EXISTS\n");
    }
} else if (!strcmp(expression, "checkd")) {
    printf("CHECK DIR\n");
} else if (!strcmp(expression, "checkr")) {
    printf("READ\n");
} else if (!strcmp(expression, "checkw")) {
    printf("CHECK WRITE\n");
} else if (!strcmp(expression, "checkx")) {
    printf("CHECK EX\n");
} else {
    printf("NOOOO\n");
}
}

However, if I even change it to something like below I get an error. It seems to be failing at the sscanf call. But that works when I don’t add some variable.

void condition(char *command) {
int flag;
printf("CAMMAND: %s\n", command);
char *cond;
char *expression;
char *file;
char *buffer;
buffer = malloc(200 * sizeof(char));

sscanf(command, "%s ( %s %s ) %[^\n]", cond, expression, file, buffer);

printf("COND: %s\n", cond);
printf("EX: %s\n", expression);
printf("File: %s\n", file);
printf("Command: %s\n", buffer);

if (!strcmp(expression, "checke")) {
    printf("CHECK EXISTS\n");
    if (file_exists(file)) {
        printf("EXISTS\n");
    }
} else if (!strcmp(expression, "checkd")) {
    printf("CHECK DIR\n");
} else if (!strcmp(expression, "checkr")) {
    printf("READ\n");
} else if (!strcmp(expression, "checkw")) {
    printf("CHECK WRITE\n");
} else if (!strcmp(expression, "checkx")) {
    printf("CHECK EX\n");
} else {
    printf("NOOOO\n");
}
}

Can’t seem to figure out why this is. Any help would be great!

Answer 1

You are getting memory corruption because you are passing pointers to characters, rather than pointers to character arrays, as the destination for %s conversions. expression, as defined, is uninitialized and points to some random memory location. Here is the relevant part of the sscanf documentation:

s      Matches a  sequence  of  non-white-space  characters;  the  next
       pointer must be a pointer to character array that is long enough
       to hold the input sequence and the  terminating  null  character

You need to allocate some space for expression etc.