C - fopen() doesn't work (returns null pointer)

Question

Why does fopen() return a null pointer?

I'm trying to save the parameters after BRIDGE and after LAN (respectively 4 and 5) in cont_br and cont_lan, but fopen() doesn't work...

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

void read_file(char file[]) {

    char cont, *str, *ctrl_str;
    int x=0, cont_br, cont_lan;

    FILE *file_stream;

    if (file_stream = fopen(file, "r")) {

        while( !feof(file_stream) ) { //Check the file dimension
            fgetc( file_stream );
            x++;
        }
        x--;
        str = (char*) malloc(sizeof(char) * x+1);
        fseek(file_stream, 0, SEEK_SET); // Return at the beginning of the file
        fread(str, x, 1, file_stream);
        char delims[] = "#";
        char *result = NULL;
        result = strtok( str, delims);
        while (result != NULL) {
            if (result == "BRIDGE") { //Check the different "blocks" in the txt file
                result = strtok(NULL, delims);
                cont_br = atoi(result);
                printf("Number of bridges: %d\n", cont_br);
            }
            result = strtok(NULL, delims);
            if (result == "LAN") {
                result = strtok(NULL, delims);
                cont_lan = atoi(result);
                printf("Number of Lan: %d\n", cont_lan);
            }
            break; //
        }
    }
    printf("Error: can't open the file! errno: %d\n", errno);
    fclose(file_stream);
}

int main() {
    char file[] = "Config.txt";
    read_file(file);
    return 0;
}

And this is the Config.txt file:

BRIDGE#4#
LAN#5#
192.168.0.1
192.168.1.1
192.168.2.1
192.168.3.1
192.168.4.1
#
LINK#
B1:3000,L1
B1:3001,L2
B1:3002,L3
B2:3000,L4
B2:3001,L5
B3:3000,L1
B4:3000,L3
B5:3000,L2
B2:3002,L3

print the path of `file` just before you call `fopen`, I guess it is not getting the path. — someone, Jun 06 '14 at 12:41
You are using feof incorrectly: http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong/5432517 — William Pursell, Jun 06 '14 at 12:44
Probably `fopen` never returned NULL; you only assumed it did, based on faulty logic elsewhere in your code. First of all, put `printf("Error: can't open the file! errno: %d\n", errno);` in an `else` block. Secondly, do not use `==` to compare strings; use `strcmp` instead. — Ruud Helderman, Jun 06 '14 at 14:11

score 3 · Answer 1 · edited Feb 20 '15 at 22:49

3

Whenever fopen fails then print the error number (errno).

#include <error.h>

You should include the standard errno.h and print the value of error in your code. Then look at the error code and find the reason.

Example

#include <stdio.h>
#include <errno.h>

int main()
{
    errno = 0;
    FILE *fb = fopen("/home/jeegar/filename", "r");
    if (fb==NULL) {
        printf("Error %d \n", errno);
        printf("It's null");
    }
    else
        printf("working");
    }

This way if fopen fails then it will set the error number. You can find those error number lists in fopen.

edited Feb 20 '15 at 22:49

Peter Mortensen

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answered Jun 06 '14 at 12:38

Jeegar Patel

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Error messages should be useful. `fb = fopen( path, mode ); if( fb == NULL ) { perror( path ); ... ` – William Pursell Jun 06 '14 at 12:45
2

`printf` can modify `errno`, so calling printf before using errno is a mistake. – William Pursell Jun 06 '14 at 12:46
how printf can modify errno? – Jeegar Patel Jun 06 '14 at 12:48
`errno` is global. @Mr.32 – alk Jun 06 '14 at 12:51
perror("Config.txt") prints Success... – YoungOldMan Jun 06 '14 at 12:55
`perror` prints the error message. When errno is 0, the related error message is "Success". If you are doing `printf("its null")` followed by `perror`, then you are seeing the manifestation of printf changing errno. – William Pursell Jun 06 '14 at 20:16

C - fopen() doesn't work (returns null pointer)

1 Answers1

Example

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