CODEWITHSUNDEEP
csegmentation-fault
Brian Beauregard
Brian Beauregard

Reputation: 1

Malloc attempts to address 0x0?

I'm working with dynamic memory in C. I'm writing a small function which should

  1. take a string (a char*) as a parameter;
  2. allocate memory with malloc for that string, copy it into the memory; and
  3. return the newly allocated string.

My program gives a segmentation fault.

char *new_line(char *line) {
        char *temp_line;
        int line_len = strlen(line)+1;
        temp_line = malloc(line_len * sizeof(char));
        printf(temp_line);
        strncpy(temp_line,line,line_len);
        return temp_line;
}

According to GDB, my program fails on this function, because apparently it is trying to do something (?) to the address 0x0. This is what GDB says:

#0  0x00007ffff7b7c3c1 in __strlen_sse2_pminub () from /lib64/libc.so.6
#1  0x00000000004008fc in new_line (line=0x0) at myfile.c:35

Can anyone offer some insight on why malloc seems to be returning or messing with 0x0, based on my function? I can't see it.

Upvotes: 0

Views: 2563

Answers (3)

MayurK
MayurK

Reputation: 1957

There are following issues.

  1. The major issue is that you are printing new string before copying data to it. The allocated memory will have some random data.
  2. It is better to check whether memory allocation is successful. Also check the input pointer.

  3. strncpy() will not put '\0' at the end. You need to put it explicitly. Since, in this case you have allocated memory by checking size of input string, you can call strcpy().

    char *new_line(char *line) {
        char *temp_line = 0;

        if(0 == line)
        {
            printf("Input is null\n");
            return 0
        }

        int line_len = strlen(line)+1;
        temp_line = malloc(line_len * sizeof(char));
        if(0 == temp_line)
        {
            printf("Failed to allocate memory for new string\n");
            return 0;
        }

        strncpy(temp_line,line,line_len);
        temp_line[line_len-1] = '\0';
        /* You can replace above two lines with strcpy(). */

        printf("Inp string[%s] new string[%s]\n", line, temp_line);

        return temp_line;
}

Upvotes: 1

John Kugelman
John Kugelman

Reputation: 361869

#0  0x00007ffff7b7c3c1 in __strlen_sse2_pminub () from /lib64/libc.so.6

It's strlen that's crashing, not malloc.

#1  0x00000000004008fc in new_line (line=0x0) at myfile.c:35

Notice it says line=0x0. That means line is NULL. The code that calls new_line is passing a null pointer.

int line_len = strlen(line)+1;

Which in turn causes strlen to be given a null pointer.

Upvotes: 2

Craig Estey
Craig Estey

Reputation: 33621

printf uses strlen internally.

You're calling printf on a buffer from malloc that has [possibly] random characters in it [it does not initialize the bytes]. So, strlen may just "keep going".

Move the printf after the strncpy

Upvotes: 1

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