Segmentation Fault while trying to read next line/character from file

Question

I have a school project where I need to read .ppm file in C and store it in a struct to be used for later tasks. Once I get the first line and assign it to a struct variable, I get errors if I try to navigate through the file again. Here is my code:

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

typedef struct {
    int r, g, b;
} pixels;

typedef struct {
    int width, height, maxColors;
    char *format; //PPM file format
    char *comments;
    pixels *pixels;
} PPM;

PPM *getPPM(FILE * f) {

    // Check if the file is valid. If not then exit the program
    if(f == NULL) {
        perror("Cannot open file: ");
        exit(1);
    }

    PPM *pic = malloc(sizeof(PPM));
    // If memory allocation fails, exit the program.    
    if(!pic) {
        perror("Unable to allocate memory for structure");
        exit(1);
    }

    // Store the first line of file into the structure
    char *fileFormat;

    if(fgets(fileFormat, sizeof(fileFormat), f) != NULL) {
        // If it is a valid .ppm format, then store it in the structure
        if(fileFormat[0] == 'P') 
            pic->format = fileFormat;
    } else {
        perror("Unable to read line from the input file");
        exit(1);
    }

//Errors start here
    int c = getc(f);
    while(c != EOF)
        c = getc(f);

/*
    char *comments;

    if(fgets(comments, sizeof(comments), f) != NULL) {
        printf("%s\n", comments);
    } else {
        perror("Unable to read line from the input file");
        exit(1);
    }
*/
    fclose(f);
    return pic; 
}



int main(int argc, char **argv) {

    PPM *p = getPPM(fopen(argv[1], "r"));
    printf(" PPM format = %s",p->format);
    return 0;
}

I have tried getting individual characters from the file. I have tried reading the whole line using fgets like I did in the previous step (for fileFormat), but every time it gives out segment fault. I have tried looking at other examples but I cannot figure out the problem. I have been at it for hours, so any help would be greatly appreciated!

Could the problem with the way the memory is allocated? Or do I need to give some sort of pointer to the file when I try to read a new line? I tried finding an answer in man pages but I couldn't figure anything out.

P.S. the while(c != EOF) { c = getc(f); } and the next commented step was just to see if it was working or not. I want to put all the information from the .ppm file into the PPM struct.

Answer 1

You're reading into an uninitialized pointer, so this is going to crash. You need a buffer:

char *fileFormat = malloc(SIZE_OF_FILE_FORMAT);

Additionally sizeof(fileFormat) returns the size of the pointer which in this case is not what you want. You need the size of the buffer the pointer points to.