no outprint from function

[kristy]

I'm having an error in my code I can't figure out. At first I thought it was an error on malloc() or sysctl(), but it seems it's neither. Since I'm all out of options, I'm posting it here.

int main() {

    char *systemName;

    printf("system information:\n");
    systemName = sysName();
    printf("\toperating system:\t\t%s\n", systemName);
    free(systemName);

    return 0;
}

char *sysName() {
    
    int mib[2];
    size_t length;
    char *osName;

    if((osName = (char *)malloc(50)) == NULL) {
        printf("Error on malloc(), no memory allocated\n");
    }

    memset(osName, 0, 50);

    mib[0] = CTL_KERN;
    mib[1] = KERN_OSTYPE;

    length = sizeof(osName);
    
    sysctl(mib, 2, &osName, &length, NULL, 0);
    
    error(); // a function that checks errno

    return osName;
     
}

The operating system name is not printed out; the line is just blank. If I set osName to be, i.e. 'A': osName[0] = 'A' the A is printed out. This leaves me thinking it's something wrong with sysctl(), but I've used the function the exact same way in another program - where it works! Can anyone see what's wrong? Help appreciated.

[Salem]

> length = sizeof(osName);
You're doing sizeof on a pointer, which is probably not what you want, if you're looking for something with a value of around 50

> If I set osName to be, i.e. 'A': osName[0] = 'A' the A is printed out
Probably because it now fits in the 4 bytes you've told it exists, rather than the 50 you think exists.

size_t length = 50;
osName = malloc( len * sizeof *osName );
memset(osName, 0, len);

[kristy]

Thanks for your reply, Salem.

I fixed some of the code, but it now produces a segmentation fault instead. I must have done something wrong (again). I'm not very familiar with malloc; but is the code correct now? It looks kinda.. weird.

char *sysName() {
    
    int mib[2];
    size_t length = 50;
    char *osName;

    osName = malloc( length * sizeof *osName );

    memset(osName, 0, length);

    mib[0] = CTL_KERN;
    mib[1] = KERN_OSTYPE;
    
    sysctl(mib, 2, &osName, &length, NULL, 0);
    
    error();

    return osName;
     
}

[Salem]

> sysctl(mib, 2, &osName, &length, NULL, 0);
Knowing nothing about this call, I would have said
sysctl(mib, 2, osName, length, NULL, 0);

[kristy]

this produced the output I wanted: sysctl(mib, 2, osName, &length, NULL, 0);

Thanks a million, you saved the day.