使用fgets进行segfaulting，即使fopen在C中不返回NULL

Question

I've looked all over and made sure there were no warnings, but my code to replace text with digits keeps returning segfault. Any help?

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

int main(int argc , char *argv[])
{
    FILE *file;
    file = fopen(argv[1] , "r");
    char *line = malloc(1024);
    if(file != NULL)
    {
        while(fgets(line , sizeof(line) , file))
        {
//things
        }
    }
    else
    {
        printf("ERROR: %s NOT AVAILABLE" , argv[1]);
    }
    return 0;
}

Answer 1

Replace:

char *line = malloc(1024);

with:

char line[1024] = {0};

or:

char line[1024];

if you don't want to clear out the line buffer.

Otherwise, you end up with two problems.

First:

sizeof(line)

returns the size of the pointer (4 or 8 bytes). That's not what you want.

Second: You have a memory leak because you don't free the line pointer at the end.

You can use malloc if you want, but you want to write clean(er) code to do this. You might do something like:

#define MAX_LINE_LENGTH 1024

/* ... */

char *line = NULL;
line = malloc(MAX_LINE_LENGTH);
if (!line) {
    fprintf(stderr, "Error: Could not allocate space for line buffer!\n");
    exit(EXIT_FAILURE);
}

FILE *file = NULL;

/* Avoid undefined behavior by making sure filename argument holds a value */

if (argv[1])
    file = fopen(argv[1] , "r");

if (file != NULL) { /* You could also do "if (file) { ... }" */
    while (fgets(line, MAX_LINE_LENGTH, file)) {
        /* ... */
    }
}

free(line);
line = NULL;

As a habit, explicitly initialize pointers to NULL , and check that they actually hold a value before using them. Welcome to C!