Data entry into array of character pointers in C

Question

this is my first question asked on here so if I'm not following the formatting rules here please forgive me. I am writing a program in C which requires me to read a few lines from a file. I am attempting to put each line into a cstring. I have declared a 2D character array called buf which is to hold each of the 5 lines from the file. The relevant code is shown below

#include <stdlib.h>
#include <sys/types.h>
#include <sys/file.h>
#include <sys/socket.h>
#include <sys/un.h>      /* UNIX domain header */

void FillBuffersForSender();

char buf[5][2000]; //Buffer for 5 frames of output

int main()
{

FillBuffersForSender();

return 0;
}

void FillBuffersForSender(){
    FILE *fp;
    int line = 0;
    char* temp = NULL;
    size_t len = 0;
    ssize_t read;
    fp = fopen("frames.txt", "r");
    printf("At the beginning of Fill Buffers loop.\n");
    //while ((read = getline(&temp, &len, fp)) != -1){
    while(line < 5){
            //fprintf(stderr, "Read in: %s\n", temp);
            fgets(temp, 2000, fp);
            strcpy(buf[line], temp);
            line++;
            fprintf(stderr, "Line contains: %s.\n", temp);
            temp = NULL;
           }
    while(line != 0){
        fprintf(stderr, "Line contains: %s.\n", buf[line]);
        line--;
    }

}

The line

strcpy(buf[line], temp);

is causing a segmentation fault. I have tried this numerous ways, and cannot seem to get it to work. I am not used to C, but have been tasked with writing a bidirectional sliding window protocol in it. I keep having problems with super basic issues like this! If this were in C++, I'd be done already. Any help anyone could provide would be incredible. Thank you.

score 3 · Answer 1 · answered Dec 05 '14 at 01:26

3

temp needs to point to an allocated buffer that fgets can write into.

answered Dec 05 '14 at 01:26

Paul Richter

6,154
2
20
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score 1 · Answer 2 · answered Dec 05 '14 at 01:26

1

When it comes to C you have to think of the memory. Where is the memory for a point with NULL assigned to it? How can we copy something to somewhere that we have no space for?

answered Dec 05 '14 at 01:26

Jonathan Holland

607
9
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user3386109 · Accepted Answer · 2019-05-16T19:50:59.143

In C programming, error checking is an important part of every program (in fact sometimes it seems like there's more error handling code than functional code). The code should check the return value from every function to make sure that it worked, e.g. if fopen returns NULL then it wasn't able to open the file, likewise if fgets returns NULL it wasn't able to read a line.

Also, the code needs to clean up after itself. For example, there is no destructor that closes a file when the file pointer goes out of scope, so the code needs to call fclose explicitly to close the file when it's finished with the file.

Finally, note that many of the C library functions have quirks that need to be understood, and properly handled. You can learn about these quirks by reading the man pages for the functions. For example, the fgets function will leave the newline character \n at the end of each line that it reads. But the last line of a file may not have a newline character. So when using fgets, it's good practice to strip the newline.

With all that in mind, the code should look like this:

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

#define MAXLINE   5
#define MAXLENGTH 2000

static char buffer[MAXLINE][MAXLENGTH];

void FillBufferForSender(void)
{
    char *filename = "frames.txt";
    FILE *fp;
    if ((fp = fopen(filename, "r")) == NULL)
    {
        printf("file '%s' does not exist\n", filename);
        exit(1);
    }

    for (int i = 0; i < MAXLINE; i++)
    {
        // read a line
        if (fgets( buffer[i], MAXLENGTH, fp ) == NULL)
        {
            printf("file does not have %d lines\n", MAXLINE);
            exit(1);
        }

        // strip the newline, if any
        size_t newline = strcspn(buffer[i], "\n");
        buffer[i][newline] = '\0';
    }

    fclose(fp);
}

int main(void)
{
    FillBufferForSender();        
    for (int i = 0; i < MAXLINE; i++)
        printf("%s\n", buffer[i]);
}

Note: for an explanation of how strcspn is used to strip the newline, see this answer.

Data entry into array of character pointers in C

3 Answers3