Problem in read line function

**leandroSousa** · 05-17-2010

Hi,

I am a beginner in C language and I am developing a function to read lines, but is showing some invalid characters.

Code:

char *getNextLine()
{
    int count = 0;
    char *retVal;

	if(feof(arq)) return NULL;

	free(retVal);

	retVal = (char*)malloc(sizeof(char));

	while((*(retVal + count) = fgetc(arq)) != '\n' && !feof(arq))
	{
	    count++;

		retVal = (char*)realloc(retVal, sizeof(char) * count + 1);
	}

	return retVal;
}

Show this.

----------------Test in C------------------------1
----------------Test in C------------------------2
----------------Test in C------------------------3
----------------Test in C------------------------4
----------------Test in C------------------------5
----------------Test in C------------------------6
----------------Test in C------------------------7
----------------Test in C------------------------8
----------------Test in C------------------------9
----------------Test in C------------------------10
x----------------Test in C------------------------11
x----------------Test in C------------------------12
x----------------Test in C------------------------13
x----------------Test in C------------------------14
x----------------Test in C------------------------15
@----------------Test in C------------------------16

The lines 11 to 16 show the characters strange.

what am I doing wrong?

thanks.

**MK27** · 05-17-2010

You are not null terminating the string.

Realloc() should not be used one byte at a time, this is very inefficient. Also, I would use fgets() to read lines. You can use a temp buffer in the function of sufficient size (say 4k), then malloc a pointer based on the strlen() of that, copy in, and return the pointer.

**leandroSousa** · 05-17-2010

Hi MK27, thanks for you help.

A question, the function 'fgets()' need receive the string size, but i dont know the line size.

**MK27** · 05-17-2010

Here's an example of how to use fgets() for something like this:

Code:

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

char *getLine (FILE *p) {
	char buffer[4096];
	char *r = fgets(buffer,4096,p);
	if (!r) return NULL;
	r = malloc(strlen(buffer)+1);
	strcpy(r,buffer);
	return r;
}

int main(int argc, const char *argv[]) {
	FILE *fp = fopen(argv[1], "r");
	char *line;

	while ((line = getLine(fp))) {
		printf("%s",line);
		free(line);
	}
        close(fp);

	return 0;
}

Couple of notes:

- "r" is used to check the return value of fgets and then reassigned some memory for use as the return value.
- 4096 characters is several pages of text. If your file may contain lines longer than that, you could adapt this function to use a variable length buffer:
Code:
```
char *getLine (FILE *p, int sz) {
	char buffer[sz];      
	char *r = fgets(buffer,sz,p);
```
I just reuse the same pointer in main(), but obviously you could assign to (eg) an array using a for() loop.

**leandroSousa** · 05-17-2010

I understand, thank mk27.
You were a great help.

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