CODEWITHSUNDEEP
cmallocconcatenationstrcat
Snake_A
Snake_A

Reputation: 21

C Programming - Use of strcat() causing errors in valgrind

I have a function that reads takes a file (from stdin) ,reads the first 3 lines and concatenates them.

char line[LINESIZE];
char *temp_fd = malloc(sizeof(char)*LINESIZE*3);
char *temp_sm = malloc(sizeof(char)*LINESIZE);
char *temp_nm = malloc(sizeof(char)*LINESIZE);

char temp_pc[LINESIZE];

for(i=0;i<3;i++) {
    if (fgets(line, LINESIZE, file) != NULL) {
        strcat(temp_fd,line);

        if (i==0)
            strcpy(temp_sn, line);
        else if(i==1)
            strcpy(temp_nm, line);
        else if(i==2)
            strcpy(temp_pc,line);
    }
}

I get two errors though in valgrind, and i as i understand, strcat is the problem. How to correctly allocate memory for my pointers? (LINESIZE is 60 btw)

Thank you!

Upvotes: 2

Views: 894

Answers (2)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726559

Since temp_fd is uninitialized, you should use strcpy instead of strcat the first time you go through the loop. This would copy the string, rather than trying to append it.

The reason for this is that strcat searches for the location at which to append data before copying the content. However, the content of temp_fd is uninitialized at the time when you call strcat, causing the problem.

Alternatively, you could put '\0' in the initial place of temp_fd right after the allocation, and call strcat in all iterations of the loop:

char *temp_fd = malloc(sizeof(char)*LINESIZE*3);
temp_fd[0] = '\0';

Upvotes: 3

StilesCrisis
StilesCrisis

Reputation: 16290

You aren't doing anything to clear out your buffer space before you use it.

There are two different ways you could fix it. Either would work:

  • Write a null terminator byte to the head of the buffer before using it (e.g. strcpy(temp_fd, "");)

  • Allocate with calloc instead of malloc

Upvotes: 4

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