exec segfault of trivial C program

Question

Running a trivial program within gdb, that forks and execl a client. The execl line (while inside inferior 2 in gdb) gives the process ID "is executing new program" and then immediately seg faults.

Code to that point follows:

int main(int argc, char *argv[] ) {
/* Create socket pair for communication with server, and fork/exec the server code */
  int ret;
  int fd[2];
  ret = socketpair( AF_LOCAL, SOCK_STREAM | SOCK_NONBLOCK, 0, fd );
  if( ret < 0 ) {
    perror( "Unable to create initial socketpair for qrServer" );
    return -1;
  }
  pid_t pid = fork();
  if (pid == 0) {
    char arg[4];
    bzero( arg, 4 );
    close(fd[1]);
    sprintf( arg, "%d", fd[0] );
    char* filename = "child";
    ret = execl( filename, filename, arg, NULL );

Just for the sake of completeness, child program begins as follows:

int main(int argc, char *argv[] ) {
/* Create socket pair for communication with server, and fork/exec the server code */
  if( argc < 2 ) {
    perror("Usage: argv[0] <file descriptor>");

Any ideas? I get nothing, just the seg fault itself and I get it immediately.

`arg[4]` is not big enough. Also get in the habit of using `snprintf( arg, sizeof(arg), "%d", fd[0] )` to help avoid this kind of fault. — Steve Summit, May 03 '16 at 16:15
@SteveSummit "*... is not big enough.*" probably not the case, as the expected values for the two FDs would be `3` and `4`. — alk, May 03 '16 at 16:29
@alk Evidently I'm blind this morning, better go eat something and stop commenting. — Steve Summit, May 03 '16 at 16:47
Maybe the execl is failing because "child" is in the current dir, which is not the search path, but difficult to say if this results in a seg fault without seeing more code. — Mark Plotnick, May 03 '16 at 17:25

score 2 · Answer 1 · answered May 03 '16 at 16:32

2

From man execl:

The list of arguments must be terminated by a null pointer, and, since these are variadic functions, this pointer must be cast (char *) NULL.

So this line

  ret = execl( filename, filename, arg, NULL );

should be

  ret = execl( filename, filename, arg, (char*) NULL );

answered May 03 '16 at 16:32

alk

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@Aenid37 If this helps, please let us know. If it helps, it suggests that (a) you're on a 64-bit platform, but (b) `NULL` is defined as plain `0`. (Which is perfectly legal, but leaves you vulnerable to this problem.) – Steve Summit May 03 '16 at 17:46

exec segfault of trivial C program

1 Answers1